I'll echo AS's question....why does he want to move back into the spare bedroom? Does he want to work on the M? Or is it just not as comfortable for him to be staying with a friend? Does he just miss the kids?
His intentions (and yours) are very important here. If he isn't committed to work on the M, then I think it is a waste of time to let him move back in. It's a step backwards for you.
I like AS's list on requirements, but Acc is right, you need to create your own. Transparency is an absolute must in my book...you have to reestablish trust.
Make sure that if he does want to reconcile, you both commit to figuring out what was missing in your M and work on that. This isn't just about forgiving him and him promising not to do it again. This is about figuring out why it happened and positioning yourselves as a couple in a way where your M is affair proof.
