From his point of view, Mulroney has a lot to lose if he doesn't take an immunity deal: we might make him give our $2.1 million back; he could be charged with perjury; he could be charged with taking cash while an MP; he could be charged with tax evasion for not paying GST on his $225/300K; perhaps other charges yet to be determined.
For the Canadian public to buy it, this cannot be a deal between Harper and Mulroney. We should gain wide public support through consultation with all parties, as well as with journalists, academics, bureaucrats and jurists; and we should host a number of public forums to seek support for the idea. It should be clear before immunity is offered that the country agrees with the plan, including having the unanimous support of MPs and premiers.
It would be best if we had someone of very senior stature to promote the idea. Jean Chretien would not be ideal as he was Mulroney's political rival (and in some Canadians' minds, Chretien is tainted by scandal himself). It can't really be a Conservative, because that would raise charges of Conservatives trying to help their own interests.
Bob Rae would be great... if he weren't the Liberal candidate in an upcoming by-election. How about Lloyd Axworthy, Louise Arbour or Michael Lewis? How about asking someone outside Canada, like a former UN Secretary-General or Nelson Mandella? Nelson Mandella's use of truth and reconciliation to help South Africa move on after apartheid is of great importance to the world; as I have argued before, it's a technique that should be used much more widely, including in wrapping up the US invasion of Iraq. This might be an opportunity for him to generalize that brilliant technique for healing national psyches after wrongdoing. Being very elderly, he can't be expected to spend a lot of time on the matter, but he could propose the framework of how the process would work to the public.