There is only one scenario where the opposition doesn't get what it wants on the detainee file. While the opposition negotiates with the government, I would hope they are also negotiating with themselves. Harper's best hope to divulge as little as possible, is to siphon off one opposition party and cut a side deal. It's imperative that the Liberals, NDP and Bloc maintain a united front, because it is this posture that will extract the most desirable outcome.
I'm not against Iacobucci being retained in some capacity, if he answers to Parliament and not the PMO. There is a certain rationale to having third party investigation, because nobody disputes that national security is an issue, weaved within the "culture of deceit". Reasonable Parliamentarians accept some tension, so if Iacobucci were truly independent and his direction clear, that might be acceptable in a certain sense. Of course, this approval assumes that Parliament is in the "loop" so to speak, that's a critical distinction. HOWEVER, any endorsement of this compromise must be acceptable to ALL three opposition parties, any deal that leaves certain parties out is a semi-victory for Harper and the extraction will be lessened with division.
The opposition should huddle after each meeting with the government and ensure they are on the same page through this process. I have been thoroughly impressed with the united front to date, and truth be told we wouldn't be in this place today, if not for a common working spirit. Now that we've reached the 11th hour, don't let political posturing usurp fundamental considerations, and this includes all sides. I would proceed with the premise that any "solution" must be acceptable to all opposition parties, if it isn't, then NO deal. That posture will provide the best outcome.