So at this point I have migrated my calendar data to my own DAViCal installation (which is mostly working except for sending/receiving invites - gotta figure out what's going on with that), deleted both of my Apps accounts, and also deleted a bunch of GMail accounts which I had lying around which had somehow (and rather creepily) gotten linked to each other anyway. I still have one gmail account for Android Market and I see no reason to get rid of that just yet, and I've relinked my YouTube account to that since there's no way to have YouTube without a Google account anymore.
It's kind of scary to be without a safety net here but on the other hand, it was also kind of scary about what sorts of stuff Google was doing in the background without my knowledge as well. (I mean, for a time, trying to log on to my YouTube account to relink it was logging me on to a completely different gmail account that I'd forgotten about, and it was insisting that I needed to create a new YouTube account as part of it! Very broken in stupid ways. But once I deleted that old gmail account, that freed up the legacy YouTube account to be relinked. Puzzling all around, though.)
Many of my friends who were on G+ with pseudonyms that hadn't gotten nuked yet went ahead and deleted their accounts too. We're all back to using LiveJournal and email.
Oh, and for a while I was thinking I'd made a mistake since all of the CalDAV clients I found for Android were standalone calendar apps (rather than CalendarProvider implementations as they should be), but finally I found CalDAV-sync which seems to be working quite nicely. It's $3 and still alpha, but that's worth it (hopefully they don't get nuked from orbit for no reason like Better Android did). They also have a CardDAV provider as well, for when I finally remove my addressbook from the one gmail account.
One missing thing for this is the ability to subscribe to friends' shared calendars, but I can still do that via my Android gmail account for now.
Hopefully there will eventually be some turnkey F/OSS suite to make it easy to set up this stuff. Email is well-known and so on, but CalDAV is still sort of nascent as far as interoperability goes.
Oh, and there's probably still a few Google account remnants out there. My apps accounts' associated Google accounts are still active (with no way to shut them off), and since I have FeedBurner and AdSense/AdWords associated with one of them, that'll probably continue to exist for a while. Which reminds me, I must get rid of all the AdSense on my site... (I've made like $29 over all time on Google page ads, so I mean, yeah. No big loss.)