You do realize the MC has already fucked Kelly, in Act 1 after Lacey came back he'd be fine if he had done at that point, even if he had felt bad for some time, what pain he'd endured would have been the pale shadow of the pain he had endured when she cheated on him, and allow her to have an actual earned moment of character growth, so it would have worked fine at the time and could have led to a far better narrative and characterization.
But the Dev wanted endless drama so now we are 3 Acts neck deep in drama, and now their is no choice but either at worst another unearned twist in character personality to fix those issues, or have some external metaphysical force to make it happen, their is no way to make that fix fit, the MC's will has been crushed entirely, he has been forced to passively just sit back and let Lacey do anything she wants.
That may be, I am just speaking to a specific solution for a Lacey "dealing with what she has dealt". We may be too far past that due to the MCs break and health, but... maybe not in terms of lacey FULLY coming to terms and this may be a way (for the route to repair their relationship) to give her the means and motivations to actually "help" rather than the "hindering" she has been doing.
I don't think Lacey will "get it" by simply having to experience him being with another, even if it is watching...
Nope... it has to be soul crushing. When I say soul crushing, I mean... every fear, every single thing that would be a horror to see and hear from the MC, every element to reflect back at her how she has been, what she has done, but wrapped up into that presentation in some way.
She literally has to be on the verge of demanding that she can kill herself at that very moment, to escape the very horror she just witnessed... This is why it should be over a course of a couple of days before they break the truth, something where they refuse to let her act, to force her to simply sit... cry, scream, sob and swirl in it.
Unless we a going for some other type of ending (demoralized, breakup, etc...) this would be a "reasonable" means to provide an eventual reconnection.
Lacey is the problem right now. Sure, she forgave herself, but she has only done so "partially". She blames herself, but she still shifted blame on the exes, on mia (which is fair), but ultimately... the reason things are the way they are aren't because of her college days (they play a big part), but because of the way she acted since coming back to the MC (and the various lies she did it under).
She hasn't come to terms with it, sure... she talks the talk, says all the right words, but she doesn't truly accept the responsibility of them. They are... simply telling the MC what he wants to hear, but not understanding what it is she is actually saying.
This type of shock could be the catalyst to that understanding, a true revelation that causes her to freak the hell out, and to be honest... I have yet to see her truly attend to it (very briefly with the split personality break). She hasn't hit rock bottom, she is nowhere near on that level of understanding.
once she does though... then she can begin to actually grow, not through words, but a show of actions to the MC. Simply being a good wife, considerate of her words and actions, of him... If she did that from the start, I doubt things would be as bad as they are (even the MC said this, if she would simply stop hurting him) and she hurts him because she doesn't understand that she is hurting him... I mean. TRULY understand.
I think from that, it would be possible for her to then work on his mental health issues. Without her being the cause of continued issues, they simply have to repair from what has already occurred and I think the MC (at least he was) was pretty good at working to that, he had a long way to go for sure, but the long road is really... the only road. There is no quick fix here, just consistency to a cause and that will eventually heal things, and allow stability over time I would think (along with a crap load of therapy both individually between them and eventually group wise).
I think it is possible, and the professor could still throw in some twists and drama as it is progressing, providing Lacey herself is no longer the cause and she becomes more a means for them to build a defense together.
Again, this is the "They end up together" resolution, the others are easy to progress in. This would be the hardest to reason to work.
Oh, there is the other side to this as well... she breaks, the "old lacey" returns (because she no longer has anything left to be the new lacey for) and this could open up a complete shit show of drama as the girls try to get her to realize it was a fake (I get the whole exorcist vibe from this, "Lacey is no longer here") ... but that may be too much... lol.. hell.. I am spit balling into dark drama now.
Oh well... they are just thoughts... I wonder what the professor will come up with though.