As I see it, poly will be a hot subject to touch because it led to a scary moment that almost cost you your marriage. I also speculate that the blocks against analyzing the ex too closely will persist because you'd have to be aware that whatever the ex did for so long (so many years), she must have had some kind of tacit go-ahead (from both Matt and you? I can't say) to do them.
But I don't see these things as crucial or urgent. The past is receding into the past, and you have plenty in the present to work on. I guess I recommend small amounts of thoughts on your therapist's assignments, but not to the point where you're trying to force out the answers or dig deeper with dynamite. Let the questions be in the stew to just simmer is what I say.
Of course, I am not the professional in this area.