Turns out downdating messes with App store. Likely iTunes Store and App store share some code modules.
With iTunes 10.4.1 running, App store downloads reliably fail. If I
also install iTunes 12.1, reboot, I can finally download that
promo-coded copy of Civilization V.
Of course to have two copies of iTunes in the App folder at the same time, one of them needs to be renamed "myTunes" or similar. They'll still share the
same prefs file, which points to the music lib, so things get complicated.
For those who'd like to try:
Old iTunes Version installers are
here.
Basic downdate protocol is
here.
Scripts I wrote to save and restore play count, rating etc, to older versions of iTunes are
here.
Doug's AppleScript to save all abum art is
here.
Pacifist is
here. (The Dev seems to have exceeded their bandwidth limit for July)
I didn't use Appzapper as recommended in the "how to downdate" article; simply did a root level file search for "iTunes" with
Find Any File and deleted anything that looked like it might be bothersome.
Pacifist install went cleanly for any version 9 point something and up. No warning messages upon iTunes startup.
I think that covers all the important links. If you care to try it yourself, make lots of backups and lots of little testlibs before leaping in whole hog. Things
will go wrong that are either annoying, or nontrivial to fix. Still, I'm happy running iTunes 10.4.1 under OS X 10.10.4. It plays my music, and does not try to sell me something every time I turn around.
As I said before, trouble arises when you try to use App Store (haven't checked iTunes store). It
really wants a current version of some code or other.
Working on workarounds for that. It looks to be possible, but if I can get it neat enough for general consumption, I'll document that in a fresh thread here.