(1) Remember that I ran Startup Repair before I used Set ID to relabel the EFI partition. I suspect that it just failed because it couldn't find an EFI partition at all (and thus had nothing useful to dump to the logfile). Now that the system is (apparently) working properly, I'm inclined to leave well enough alone.
(2) Curiously, the disks have swapped again - the system drive is now Disk3 and the other NVMe drive is Disk4. There are definitely 9 partitions counting from Disk 0 to Disk 4 though (see below screenshot), so that's consistent with BCDEdit's labeling from yesterday. Reminder: Disk0 is a SATA SSD, Disks 1 and 2 are SATA HDDs, Disks 3 and 4 are M2 NVMe.
Though, even more curiously, BCDEdit still thinks that it's partition 9 today, despite the system drive being Disk 3 again:
I haven't rebooted since yesterday though, just let it come in and out of sleep mode.
(3) I agree, but for now I want to wait and see what happens with future updates. Also, even though there aren't a lot of things to reinstall, it's still annoying enough that it's worth avoiding. If 1903 has issues installing, then it's easy enough to download the media creation tool again and then wipe/reinstall directly to 1903, and I can create the recovery partition then.