That's a good idea to save the old key. I added this to my instructions.
BTW, I forgot to mention that I restarted after the registry edits, before I retried the update successfully.
Here is how I solved the 0x800f0922 problem:
TL;DR (but read the full explanation below, to make sure you have the same problem that I encountered):
(1) go to HKLM/software/microsoft/windows/currentversion/winevt/channels, and delete "Microsoft-Windows-Store/Operational". Might be a good idea...