Notwithstanding Greg's mean comment on my last post, it looks like I will indeed be able to retrieve everything. It will just be a fairly long semi-manual process where I have to reimport each old mailbox one at a time rather than grabbing a bunch at one time (because mail.app spins for an hour or two then crashes when I do that). And there is still a chance that I will find a corrupt mailbox in the mix somewhere. But I think I already verified that the ONE mailbox that would have had content that wasn't duplicated elsewhere (in either a day old partial backup I had, or a two month old full backup I had) is fine. I may still be missing something, but I think at this point I feel confident I can retrieve everything, it will just take some work. I'll do a little each evening and then try to finish up on the weekend I think.
In the mean time, I'm still not checking email regularly.