(CNN) When Vice President Mike Pence departed a September meeting with the Ukrainian President, the US ambassador to the European Union moved quickly across the room on the mezzanine level of the Warsaw Marriott to corral one of the new President's top aides.

After a meeting in which President Volodymyr Zelensky expressed concern over the status of US security assistance to Ukraine , Ambassador Gordon Sondland had a possible solution: publicly announce investigations into President Donald Trump's political rivals and the hold on the aid would be released.

It was a meeting that Sondland, the US ambassador to the EU, left out of his initial deposition in front of House impeachment investigators -- and only came to light after Sondland amended that testimony. The episode was, however, described in more detail by others who have testified since, including one White House official who was so unsettled that it was reported to the White House counsel.

The meeting -- and its halting disclosure -- underscores why Sondland, a hotel magnate who had little political and zero diplomatic experience but could seemingly get Trump on the phone when needed, has become a central witness in the House's impeachment inquiry -- and also its most problematic.

But Sondland's testimony also creates issues. The deposition itself is riddled with contradictions or recollections that differ from those of other witnesses. Sondland failed to mention conversations with the President that others later recounted.

Beyond Sondland's need to amend his testimony, several officials questioned what they perceived to be his inflated sense of self. In conversations with other officials, Sondland bragged about his connections to Trump and regularly touted -- for effect -- his ties to acting White House chief of staff Mick Mulvaney. In his testimony, he minimized his connection to both the President and his top aide.

Sondland's attorneys have declined to comment on many of the specific developments that appeared to contradict his testimony.

"Ambassador Sondland looks forward to testifying next Wednesday," his lawyer Kwame Manley said Friday.

In his amended testimony, Sondland said that he "presumed" that the freezing of aid was linked to announcing an investigation -- a clause that Republicans have seized on to argue Sondland was only stating his opinion. It remains unclear whether he was operating unilaterally or under explicit orders from Trump.

One thing is certain, however: Sondland has the most questions to answer when he sits at the witness table, by himself, in front of the House Intelligence Committee on Wednesday. And his knowledge -- much of it first hand -- has the potential to reshape the direction of the impeachment proceedings against Trump.

Gordon Sondland, US ambassador to the European Union, arrives to the Capitol for his deposition as part of the House's impeachment inquiry on October 17.

'An op-sec nightmare'

Sondland came to his post as EU ambassador after a career founding a hotel chain and as a major donor to the President's 2016 campaign.

Sondland is described by other officials as having often rubbed career foreign policy hands the wrong way. It was the way he operated in his portfolio that created the most heartburn -- and landed him in the middle of the ongoing investigation. Former White House Russia expert Fiona Hill clashed with Sondland, though she did defend his intentions.

"He was clearly, you know, a savvy guy," Hill told lawmakers during her closed-door deposition . "He's charming. He's funny. He was well-meaning." She recounted his efforts at the embassy, including reports of significant spending on upgrades, and said she viewed them as positive efforts to boost morale.

Fiona Hill, former deputy assistant to the President and Senior Director for Europe and Russia on the National Security Council staff, leaves after reviewing transcripts of her deposition at the US Capitol on November 4.

But she and others also recounted that the same gregarious nature had serious potential pitfalls.

One source who works closely with Sondland told CNN he is "an op-sec nightmare" given his penchant for using his personal phone, calling repeatedly on insecure lines, texting and using What's-App regularly and emailing at will.

"He was also doing this in a way that I thought put him at risk," Hill said.

Hill recounted that Sondland would pass out her number, and the numbers of others on the White House staff, to foreign officials who would then show up at the White House unannounced and call for entrance and a meeting.

In another instance, the National Security Council's top Ukraine expert Lt. Col. Alexander Vindman said there was a presidential delegation to Ukraine, and Sondland's name was taken off the list because there was concern he would freelance.

"It was outside of his portfolio, and he tended to go off script, so there was some risk involved," Vindman said.

None of the witnesses interviewed to this point could identify exactly how or when Sondland was given Ukraine as part of his portfolio. Sondland, in his testimony, said he was operating at the direction of former national security adviser John Bolton and Secretary of State Mike Pompeo.

Hill, who described a heated dispute with Sondland on the issue, echoed others in saying, essentially, it came down to Sondland repeatedly telling people his remit came directly from the President.

"I said, 'Who has said you're in charge of Ukraine, Gordon?' And he said, 'The President," Hill testified. "Well, that shut me up, because you can't really argue with that. But then I wasn't, to be honest, I wasn't really sure."

Given that directive, others worked to keep Sondland in the tent as two separate policy processes with regard to Ukraine emerged, but their view of Sondland was hardly generous. That became particularly problematic when it came to US aid to Ukraine. Tim Morrison , the former National Security Council aide, recounted that he'd had several conversations with Bolton on how to get Trump to sign off on the security assistance, but noted they were both "mindful in those discussions about Gordon is this free radical out there."

In the view of those in the traditional inter-agency process, Sondland may have been well intentioned, but in every way had gone off the rails.

"He'd just gone off the road," Hill testified. "No guardrails, no GPS."

Sondland had his defenders. Former US special envoy for Ukraine Kurt Volker testified that his role was very helpful.

"I found his engagement to be very useful," said Volker, who is scheduled to publicly testify Tuesday afternoon. "He's a political appointee and had close ties with the political side of the White House that I did not have."

Asked if Sondland had a close relationship directly with Trump, Volker said, "I would say that he felt that he could call the President and that they could have conversations. I don't know how close."

Sondland arrives at the US Capitol on October 17.

A Mulvaney Pipeline

For Sondland, the quiet September 1 meeting in Warsaw with a top Zelensky aide was far from the first time the idea of investigations had come up.

In fact, according to several White House officials, sources involved and text messages released by investigators , it had been an idea in the works for months -- and it was an idea that had its roots in Sondland's relationship with Mulvaney.

Sondland himself told investigators he hardly had a relationship with Mulvaney.

"All I can tell you is Mulvaney was almost impossible to get a hold of," he said. "He rarely responded to emails and almost never returned phone calls."

That runs contrary to the testimony of multiple witnesses.

"He had a pipeline," testified Vindman, who added Sondland would often go directly to both Muvlaney and Bolton, unlike other ambassadors.

In fact, according to Hill, as far back as an April 22 Sondland said he'd worked directly with Mulvaney to edit an initial draft of a congratulatory letter to Zelensky for his election to mention the desire for Trump to meet with Zelensky.

That meeting still hadn't occurred when top Zelensky aides came to the White House on July 10. After a formal meeting, the group moved off to the Ward Room in the White House for a smaller group session.

That was a surprise to Hill, who noted that given the room's location near the Situation Room, it was rarely a place foreign nationals would be sent for an informal meeting.

But Sondland told Hill that he'd requested the room directly from Mulvaney's office, she testified.

It was at that meeting where Sondland pressed the Ukrainian officials about investigations -- citing Burisma explicitly, Hill told investigators -- in exchange for a Zelensky meeting at the White House. That exchange about investigations for a meeting with Trump had been explicitly put together with Mulvaney, Sondland told those in the room. It was also reported by multiple people to John Eisenberg, a lawyer on the National Security Council, the officials testified.

"To the best of my recollection, he did specifically say 'investigation of the Bidens,' " Vindman said.

Hill described Bolton abruptly cutting the meeting short when Sondland raised the investigations, and later told Hill he was "not part of this drug deal that Sondland and Mulvaney are cooking up."

Sondland's view was starkly different: "I thought it was a great meeting and we all left happy."

Sondland is seen outsidethe US Capitol on October 17 in Washington, DC.

'Maybe he could even be the hero'

Sondland's view, according to one person who worked with him directly, was that he needed to deliver for Trump.

"He saw this as a deal that could be made, and maybe he could even be the hero if the assistance went through," the person said, stressing that the end-game goal for Sondland -- bipartisan support for Ukraine, with access to the full security assistance -- was the same as that of the White House officials who disagreed with his tactics.

What isn't clear, this person said, is whether he had been told directly by Trump that a deal regarding investigations needed to be made, or that Trump had just repeatedly discussed the investigations, and it was Sondland himself who decided it must be tied to the security assistance.

In his deposition, Sondland downplayed his connections to Trump. Asked about touting the special assignment he was given by the President, Sondland responded he was "spinning a little."

Sondland describes his calls with Trump as "very" limited, listing out several, including his September 9 call where Trump told him there was "no quid pro quo" and before he headed to Kiev to meet with Zelensky on July 26.

Others came away a different view. Bill Taylor , the top US diplomat in Ukraine, testified both privately and publicly that Sondland told him that Trump wanted to put Zelensky "in a public box" by announcing the investigations, and that everything, from the security aid to a White House meeting, depended on the announcement.

Taylor also testified about Sondland's call with the President on September 9, in which the President says there was "no quid pro quo," but Taylor added that Sondland told him the President also said Zelensky had to "clear things up and do it in public."

According to Sondland's testimony, he wasn't told explicitly by the President that he wanted Ukraine to announce an investigation, but rather from the President's personal lawyer Rudy Giuliani.

"I heard that from Rudy Giuliani," Sondland said. "I never heard it from the President. I am assuming Rudy Giuliani heard it from the President, but I don't know that."

US Ambassador to the EU Gordon Sondland, at left, and European Commission Vice President Maros Sefcovic talk with to reporters aboard Air Force One in May in Louisiana.

A new call raises new questions

Sondland testified to several calls he had with Trump, but the list left out a key call Taylor revealed in public testimony last week: Sondland spoke to Trump following a meeting with Zelensky that occurred the day after the July 25 call where Trump asked Zelensky to investigate the Bidens.

That call was revealed in testimony from Taylor and his aide, David Holmes , who told lawmakers that he was able to overhear Sondland's call conversation with Trump because Sondland held the phone away from his ear in a restaurant. In the call, Trump asked Sondland if the Ukrainians were going to "do the investigation," and Sondland responded, "he's gonna do it," according to a copy of Holmes' opening statement obtained by CNN.

Following the call, Holmes testified he asked Sondland why Trump doesn't "give a s--t about Ukraine?"

Sondland responded that Trump only cared about "'big stuff,' that benefits the President, like the Biden investigation that Mr. Giuliani was pushing," according to Holmes testimony.

Sondland testified that he was unaware until much later that Trump had referenced Biden on the call with Zelensky, and it did not come up at his July 26 meeting with Zelensky. But Holmes' conversation with Sondland raises yet another question about when Sondland discovered that the push for investigating Burisma, the Ukrainian energy company that hired Hunter Biden, meant to Giuliani an investigation into the Bidens.

"I never made the connection between Burisma and the Bidens until the very end," Sondland said. "That is my testimony. I heard the word 'Burisma,' but I didn't understand that Biden and Burisma were connected."