Mulroney will have several goals in testifying:
1. Salvage his reputation
At this point, Mulroney can't salvage his reputation among people who have closely followed the Schreiber payouts. The known facts show him to have lied and perjured himself, as well as to have made a deal for large quantities of under-the-counter cash while prime minister.
But that's not game over. Mulroney is a smart man, and he is going to work like crazy to rehabilitate himself in the public eye. Given the evidence against him, this may entail muddying the water so that people who support him have a way to justify their support. But given the number of lies he's been called out on, his best strategy will be to appear to come clean and then to apologise.
2. Reduce his legal liability
In the same way that Bill Clinton tried to explain the sentence "I did not have sex with that woman," Mulroney will doubtless try to argue that "I had never had any dealings with him" was not a lie. The argument might go like this: When he said this in 1996, he was referring to a time in the past, before accepting all the cash. (Even that wouldn't be true, as they have had dealings since Schreiber bankrolled Mulroney's leadership bid in the early 80s. Unfortunately, just after Mulroney took office a mysterious series of burglaries, thought to be masterminded by his team, did away with all documentation of his leadership donations.)
Mulroney's best tactic will be to appear to cooperate fully, but he will be careful not to say anything that could further implicate him.
3. Put an end to the scandal
The most important goal for Mulroney must be to stop the negative talk about him, and to stop the investigations. He might even admit to wrongdoing and pay a fine or voluntarily return the $2.1 million - anything to stop this. (For those who think he can't afford it, think again. He and Mila have enormous collections of art, jewellery and antiques, and Mulroney has made a ton of money since leaving office.)
Given Mulroney's goals, the most lucrative line of questioning for the Ethics committee may be the cover-up. Mulroney is not going to spill the beans about his 1993 agreement with Schreiber (for one thing, it's the subject of a civil suit as Schreiber tries to get his $300,000 back). But Mulroney's interests will be served by coming clean with the Canadian public about something and apologizing - and the thing he can come clean about without subjecting himself to further litigation is the cover-up. The committee would do well to make sure they give Mulroney every opportunity to come clean about the cover-up.