Retired Supreme Court justice Ian Binnie was careful last month to avoid linking his report on Senate expenses to the Mike Duffy trial, but several of his findings strike at the heart of the P.E.I. senator's criminal proceedings

"I was very circumspect not to get into issues that would overlap or conflict with the Duffy trial. It's a totally different process," Binnie said when he released his report on "questionable" and inadmissible expenses identified by the auditor general.

A central issue in Duffy's case was the designation of the senator's home in P.E.I. as his primary residence, which made him eligible to claim meals and other expenses for his time in Ottawa, despite the fact that he has lived and worked in Canada's capital for decades.

Duffy's trial reconvenes tomorrow to hear the judge's ruling on 31 charges of fraud, breach of trust and bribery. Duffy pleaded not guilty to all charges.

Binnie was asked to adjudicate on a similar issue when reviewing the auditor's findings for Conservative Senator Pierre-Hugues Boisvenu.

The 'fiction' of primary residence

Boisvenu claimed Sherbrooke, Que., was his primary residence even though he had lived in the Ottawa region for the majority of time under consideration by the auditor general's investigation.

Binnie wrote in his report that Boisvenu had no right to claim living expenses and per diems in 2012 because he spent only 45 days at his "primary" residence in Sherbrooke, in large part due to divorce proceedings.

Binnie dismissed the claim that financial statements and a provincial health card alone — items Duffy also referred to as proof of residence in his case — were enough to determine a senator's primary residence.

"Regardless of the various registrations, Senator Boisvenu was in fact resident in [the National Capital Region] in 2012," he wrote. "If in fact the senator was not primarily resident in Sherbrooke in 2012, he ought not to receive financial allowances based on the fiction that he was."

Boisvenu's lawyer argued that there was nothing in the Senate rules that demanded a senator spend a certain number of days at their primary residence, something Binnie acknowledged was true.

Senator Pierre-Hugues Boisvenu faced many of the same questions about his primary residency that were adjudicated during Mike Duffy's trial. Former Supreme Court Justice Ian Binnie's findings on this file might be problematic for Duffy. (Jacques Boissinot/Canadian Press)

But the former justice said that a degree of common sense should be considered when filing living expense claims, saying anything else would be disingenuous.

"Primary residence is a matter of fact," he concludes. "A senator is not entitled to claim travel status when he is living at what is in fact his primary residence," adding that Boisvenu should be compelled to repay the $20,467.33 in expenses he had claimed.

Duffy's criminal defence lawyer, Donald Bayne, has tried to present a similar defence as that employed by Boisvenu, arguing that Senate rules around residency were vague at best. Duffy needed to deem his Cavendish cottage as his primary residence as it was his home in the province he was appointed to represent, Bayne argued.

Duffy claimed living expenses and per diems not because he was out to defraud the Senate, but rather to fulfil his constitutional obligations as the senator for P.E.I., Bayne said.

Duffy says he was forced to file claims

Duffy himself said in court he was told by Senator David Tkachuk — the then-Conservative chair of the Senate's internal economy committee — to file the expenses so as not to "create any light between you and any other senator who's on travel status when they're in Ottawa but most particularly any P.E.I. senator."

"He said ... to not do so could create controversy about whether or not you are truly a Prince Edward Island senator, a resident of P.E.I., you're constitutionally qualified, you must do this," Duffy said Tkachuk told him.

Senator David Tkachuk says he told Mike Duffy that if he was living in Ottawa he shouldn't be claiming living expenses and per diems. Duffy told a different story in court, insisting that Conservative leadership in the Senate pressured him to file expense claims to assert his legitimacy as a senator from P.E.I.

But in his report, Binnie said the expense claims are a matter of Senate administrative policy and are not a reflection of a senator's constitutional right to represent a province in the Red Chamber.

"This has nothing to do with his constitutional qualification to continue to sit as a senator for Sherbrooke," he said of Boisvenu's case. "The issue here is the Senate travel policy, not the constitution."

Tkachuk has challenged Duffy's testimony, telling reporters he remembers their conversation about living expenses much differently.

"I would have told them that if you're staying here in Ottawa that you should be claiming expenses, because your primary residency is in your home province. And if I was living here in Ottawa, and that was [my] primary residence, I shouldn't be claiming expenses," Tkachuk said, echoing the conclusion reached by Binnie.

Duffy faces expulsion

The speculation will stop Thursday when Charles Vaillancourt, the judge who presided over Duffy's trial, delivers his ruling.

If Duffy — who is currently on a leave of absence from the Senate with pay — is convicted of just one criminal charge and receives anything other than a discharge at sentencing, he will be suspended from the upper house, a Senate spokesperson told CBC News.

His $142,400 a year salary would immediately be cut off. The resources that are available to a senator, including his office, staff, and research budget (which has been at the heart of his criminal proceedings), would also end.

Duffy's suspension would remain in effect until all of his legal avenues have been exhausted.

If a conviction is upheld after an appeal, it would then be up to his fellow senators to decide whether a permanent expulsion is appropriate.

If Duffy is cleared of all criminal charges, he will return to work right away as if the whole matter — a three-year long legal saga — had never happened.