Hi Dan, thanks for your reply. I finally got it to update. I accomplished it in a kind of hacky way: I had to go through each update file (ud_215.php, etc) and comment out the lines that were throwing errors. This took a while, but in the end it got me through the process.
I realize now that either the DB backup I was using was one taken after an initial failed attempt to update OR it had artifacts or remnants from prior updates or something. Basically it wasn’t a completely pure pre-update-attempt version of the DB.
I would advise anyone having problems with updating (and I have noticed a lot of them in these forums) to be super-anal about backing up entire site and DB prior to any modifications or upgrade attempts. If it’s too late for such precautions, I would recommend forging ahead even if it requires the tedious commenting of lines from the update files as mentioned above.
Thanks and good luck!
PS Also, I understand that perhaps EE is more complicated than WP but wow if EE had the same kind of simple one-click upgrade functionality, I would be 100% sold for life.