I agree that 'just' being in an affair is unacceptable. You deserve better, after all you are a goddess, right? And you're right about detachment, you need to be detached enough to be ok no matter what, but not so detached that you shut the door on him forever.
So yes, accept his gifts with thanks and the kind of gratitude you'd give anyone else, but don't read too much into it, you'll only end up disappointed if it's just a temp check. At some stage he might keep giving more and more and make it obvious that he wants R rather than to check in on plan B, but he's not there yet is he? For starters you're assuming the OW is still in the picture...
