I had a very similar take as you when this started. I thought I could unconditionally love and understand my now former husband out of this. If it is a true or mild "crisis" (not illness), this may be possible.
I know people in real life who weathered this. The difference was that the affair was not crazy (i.e. they chose an affair partner who was not implausible/out of character), and they had some level of empathy and self awareness of their behavior. No one moved out for more than six months. They all dressed, talked, ate, and acted pretty much the same. ALL were seeking some form of therapy. Their spouses were patient, kept busy, waited it out. All reconciled within six months to about one year, and were able to describe their internal process during the crisis.
I would say if your spouse is exhibiting no drastic personality change apart from wanting the marriage to be over, and being less kind to you/unhappy with marriage, then it could be something more like the temporary "crisis" we all hope this is when we encounter this forum.
If he is being abusive, talking, dressing, acting differently, spending money differently, showing zero concern for you but also limited understanding of the impact of his behavior, erratic, change to physical appearance, lack of irony or self insight, I think this is something more serious than a temporary crisis.
It is beautiful, admirable, and shows the true depth of your love that even in the midst of your pain, you want to help your spouse, to find an explanation, to forgive. I hope in this you can see the beauty of your own heart.
I think a very sick/unwell MLCer is very hard to help, unless their family or friends is able to intervene in a meaningful way. Hold on to your loving and protective feelings, but extend them to yourself as well. The line you draw has to be your own emotional, physical, and financial safety.
Big hugs.