[ Warning: long rant follows ]
Well, after a week of calls to Apple and trying various things, I still have not been able to successfully restore my new iPad Pro from my old iPad Air:
1 - “quick transfer” of old to new over Wifi gets a transfer error after seeming to copy most of it.
2 - restore of iPad Pro from last (very recent) iPad Air backup on iCloud failed, or should I say, got stuck with 5 GB remaining, and repeated power off/power on actions have allowed it to transfer more data, but it finally got stuck with 700 MB remaining, and nothing gets it past that.
3 - I can’t even backup the old iPad Air to my Windows PC using iTunes. I didn’t want to go down that path, but had no choice. Using Windows 10, most recent iTunes release, Apple branded lightning to UCB-C cable, but the backup always fails after transferring about 11.4 GB with an unspecific “error reading or writing to iPad”.
4 - Interestingly, I CAN use iTunes to back up the iPad Pro, missing data and all.
Apple says it’s impossible to have multiple iCloud backups of the same device, which I think is idiotic, since it wouldn’t cost Apple anything and in fact might make people pay for more iCloud storage. My thought was to rename the old backup of the Air - which is probably corrupt, but at least it exists - and try to do another full backup to iCloud. Of course, I could delete the old backup and try to create another one, but I suspect if even an iTunes backup to a PC fails, then an iCloud backup will too, and then I will be left with not backup at all.
So at this point my options would seem to be:
1 - have some “genius” at an Apple store make a successful iCloud or local PC backup of the Air, perhaps by being able to correct whatever is wrong with the Air that’s prevent it.
2 - somehow sync my Books from the Air to the Pro, since it appears that only the Books app is missing files. The majority of them are there, but about 150 aren’t - all PDFs - and it’s difficult to even figure out which ones are missing.
I’ve read online that there are various ways of doing that sync of Books between devices, and one even uses iTunes, but that seems to depend on there being a successful backup of the Air, which I don’t have. I even tried Airdrop to transfer one of the missing books, but, unbelievably and idiotically, it transferred it with a name of “untitled 9” instead of the name I had given it, which makes it highly unsatisfactory for mass transfers. Plus, I’d still have to be able to determine exactly what books are missing, which is also difficult.
I did find one odd thing with the Notes app on the iPad Pro. All my locally stored notes - over 7000 of them - seem to be there, and I can see the 3 test notes I created in iCloud, but one of them has an embedded PDF which I can’t see: the title of it is there, and so is the size, but the file is somehow corrupt and I can’t even save it to Files for examination. The same iCloud note when viewed on the iPad Air still looks fine. Is the embedded PDF not also in iCloud storage and instead only locally on the Air? That would be a pretty stupid design if true. But I can create a new iCloud note on the Pro with an embedded PDF and it is immediately visible, PDF and all, on the Air.