I find it concerning that at five months out, 1) he doesn't want you guys to "wallow", 2) MC says you may never know his whys, and 3) you get the sense he's not going to do the work to find out why. That tells me that he might not truly change, and that your M isn't safe.
Considering especially that it took him 3 months after D-Day to finally commit to R (is that right?) he very much needs to "wallow" in it - in the sense that he needs to really spend time examining and OWNING it and then figuring out how he needs to become a healthier person and spouse.
Him needing to be healthier is reason enough... but equally importantly, how is he going to help you heal if he just wants to move on? What, he's going to leave you behind with all the hurtful memories and questions? He's gotta stay with you and help you heal from the nuclear bomb he dropped on your M. The bomb doesn't just hit and life goes back to normal... there's mourning the dead and burying and honoring them (in this case, the precious things in your M that were tainted or destroyed by his A), there's radiation poisoning to heal from (if yours is the feeling of total devastation and his is the feeling of shame, then the worst thing he can do is ignore both your feelings!), there's rebuilding the home (many times this involves relocating), and if you're rebuilding at ground zero... the ground has to be stable before you can even think of rebuilding! Does he mean to say he already has the tools to rebuild and to help you heal? Really? What if his tools consist of a monkey wrench and an oxygen tank, where what you really need are hammers, nails, and a table saw? I hope he and the MC don't "give up" too soon, it sounds like they could easily miss out on a very important transition and growth period, and you need support!!!