When Jeremy Corbyn rose to deliver his acceptance speech at the QEII Centre in Westminster on 12 September, he was greeted with roars and whoops by those who had elected him as leader. Two days later, when he entered committee room 14 to address the Parliamentary Labour Party for the first time, he was met with silence.

The antithetical receptions were evidence of his simultaneous strength and weakness, the combination that may define his leadership. Corbyn was elected with a vote share of 59.5 per cent (amounting to 251,417 votes) under the one-member-one-vote system, giving him a greater mandate than any of his predecessors. But just 14 of his fellow MPs are thought to have backed him.

Most of the 36 who nominated him, out of desire for a “broad debate”, did not support his bid. There is still resentment among Corbyn’s opponents at those who allowed him over the border. The Brownite MP Ian Austin carries a list of those who nominated the left-winger, in order to remind himself never to do them favours. Had Labour retained its electoral college system, which awarded MPs one-third of the vote, Andy Burnham or Yvette Cooper would have won. Corbyn achieved a landslide victory among members but endured a landslide defeat among his MP colleagues.

It was not only this unbalanced result that weakened his starting position. Corbyn, a backbencher for 32 years, has never served in government or in the shadow cabinet. Asked during the campaign what experience he had of management, he cited his time as chair of Haringey Council’s public works committee in the 1970s. That he never anticipated winning when he was persuaded to run as the left’s candidate makes his victory still more disorienting.

Before Corbyn’s election, moderates entertained apocalyptic visions of a ruthless left-wing seizure of Labour’s commanding heights. Max Shanly, a Corbyn supporter who sits on the national committee of Young Labour, said: “The Labour left will have to act swiftly and I am afraid brutally in many cases. The PLP will have to be brought into line. Some members of party staff will need to be pointed towards the exit.” Shanly had no official role in the Corbyn campaign but his Bolshevik rhetoric spoke to many MPs’ fears.

Yet it is amateurishness, rather than militancy, that has characterised Corbyn’s opening days as leader. Two hours after his election, news circulated that he had withdrawn from a scheduled appearance on The Andrew Marr Show on BBC1 on account of the imminent shadow cabinet reshuffle. In the event, his first act the next day was to speak at an NHS foundation trust fun day in his north London constituency. “He’s behaving like he’s still a backbencher,” an ally told me. “He needs to remember he’s leader of the opposition now.”

News channels had not been informed of the visit, denying Corbyn a chance to reach an audience beyond Islington North. Supporters speak privately of their fear that he is wasting the short time that new leaders have to define themselves. Rather than adopting a laissez-faire approach, as some expected, the Conservatives have been swift to frame Corbyn as a threat to national and economic “security”. Even more than Ed Miliband did, Corbyn risks a situation in which the public hears only the story that his opponents want to tell.

His eventual shadow cabinet reshuffle became a PR catastrophe when John McDonnell’s appointment as shadow chancellor confirmed that no women had been selected to shadow the “great offices of state” (chancellor, home secretary and foreign secretary), despite an already all-male leadership team. Angela Eagle, the new shadow business secretary, was hurriedly named shadow first secretary of state in an attempt to assuage outraged tweeters. Corbyn’s team later argued that it was health and education, held by Heidi Alexander and Lucy Powell, that were the truly great offices. As a senior MP told me, the choreography was wrong. Had Corbyn’s team announced these positions first, the damage may have been limited. It is a rich irony that the leader of the first-ever majority-female shadow cabinet (16 women, 15 men) has exposed himself to the accusation of having a “woman problem”. The lack of an adequate leader’s team, with much of the burden falling on his chief of staff, Simon Fletcher, was immediately evident. Others fear, as the new MP Jess Phillips suggested at the PLP meeting, that Corbyn has a genuine blind spot with women.

McDonnell’s appointment was provocative not only for reasons of gender equality. He is Corbyn’s ideological soulmate – and ran his campaign – but far more abrasive and divisive. “People like Jeremy even if they disagree with him,” one MP told me. “They don’t like John.” Because of his back catalogue of incendiary quotes in praise of the IRA and his 2010 joke about assassinating Margaret Thatcher (for which he apologised), McDonnell is shunned by many prepared to break bread with Corbyn.

However, others contend that Corbyn was right to appoint an MP who shares his politics and whom he can unreservedly trust. “The leader and shadow chancellor need to be in harmony with one another. It makes sense in that regard,” Jonathan Reynolds, a Liz Kendall supporter, told me.

Moderates believe that Corbyn’s choice of shadow chancellor, against the advice of the trade unions and others, will make it easier to blame him if his project fails. Other are concerned that the new shadow cabinet is too ideologically impure. After the PLP meeting, Diane Abbott complained to MPs that just three of the new leader’s supporters (herself, McDonnell and Jon Trickett) had been included. Corbyn’s appearance that evening did little to erase the impression of inexperience. The first half of his speech was nearly identical to the one he had delivered at the QEII Centre, even including the same joke about him and his fellow leadership candidates re-forming as an “Abba tribute band”. Corbyn went on to refuse to rule out campaigning for EU withdrawal, humiliating his shadow foreign secretary, Hilary Benn, who had earlier told the Today programme (while standing in for the Labour leader): “We will be campaigning to remain in the European Union in all circumstances.” To the shock of MPs, he refused to confirm that he would wear a red poppy at the Cenotaph, forcing a hasty clarification by the party’s press team.

After the shadow cabinet gathered for the first time for a photocall on Monday 14 September, they were surprised to receive a text message informing them that the usual Tuesday meeting would not take place. Nor would a meeting be held the following week. Shadow cabinet members have not been told when they will speak at the annual party conference in Brighton, which opens on 27 September, nor what the slogan and theme will be.

At the PLP meeting, as MPs raised the “red-line” issues of Trident, EU membership and Nato, Corbyn’s response was, as ever, to promise debate and discussion. Yet shadow cabinet members are unclear how decisions will eventually be taken. “Is it majority opinion, is it shadow cabinet, is it PLP, is it going to be more flexibility on whipping and more free votes?”

The first test of Corbyn’s approach will be the Conservatives’ planned vote on air strikes in Syria, an issue on which as many as 30 Labour MPs are prepared to defy their anti-war leader.

There is a general sense of bewilderment. “I kept telling myself not to underestimate the left,” a senior moderniser said. “It turns out I overestimated them.” But as one shadow cabinet member noted, Corbyn’s administrative incompetence will be little noticed by his supporters (who have been joined by more than 30,000 full members since his election). A former frontbencher told me that Corbyn should spend as little time in Westminster as possible and concentrate on building the movement that carried him to victory.

“At least nobody’s dead,” a Corbyn supporter told the MP Simon Danczuk in Strangers’ Bar in the Palace of Westminster after the PLP meeting. The internecine warfare that some forecast has been avoided, or at least delayed. Even Corbyn’s most vociferous opponents are prepared to give him time and space in deference to his mandate. But if the chaos and confusion endures, a reckoning will come soon enough.