I think it's a good idea to attend, not as MC but as IC, as long as your W signs a release that allows her IC to share anything from an IC session with you - and with a provision that the IC will call you if your W changes or cancels the release.
Also, using her IC as MC, with the release, may work, if you start with the A and your W's screw-ups, not with any pre-A issues. If you deal with the A first, a lot of those pre-A 'issues' will become moot, so it makes no sense to deal with them first.
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ETA: In theory, your IC owes a a duty to you; your W's IC owes a duty to her; and your MC owes a duty to your M. Some ICs and MCs can navigate the dual role of MC & IC to one partner, if there's a release that allows the MC to share anything from IC sessions in MC, but they're rare.
Without such a release, the C has to create boundaries in their head, and that's really difficult.
My W signed a release, so anything she said in IC was fair game for MC. Besides, she was (past tense) the problem, and I was basically an observer and fact-checker for more IC for her during our 'MC'. Also, we started with all A, all the time, and even when that stopped, I could bring up the A whenever I felt like it and my concern got immediate attention.
Attending a session or 2 could work if you used it to level-set, that is, to make sure your W told a version of the truth you can sign on to.
But if she's sold her IC a bill of goods, this may be a trap - perhaps to batter you emotionally into taking responsibility for your W's A.
If your W really wants to R, a joint session will probably help. If she's maneuvering for a D, it will hurt. Calling the C to find out the goals and method of the joint session is a good idea. I'd write down the goals so that you can complain to your state's licensing authority if you get blind-sided.
[This message edited by SI Staff at 9:19 PM, Wednesday, February 22nd]