It isn't really convenient to do that at the moment, but here is a guide that involves some manual looking around in pmaports: https://wiki.postmarketos.org/wiki/Configure_postmarketOS_for_multiple_UIs_or_users

Yes, here: https://gitlab.com/groups/postmarketOS/-/milestones/25

Haven't seen that error yet, but I'd suggest trying other Alpine mirrors (set them in your /etc/apk/repositories): https://mirrors.alpinelinux.org/

(there's also "apk fix" which helps when installing a package failed earlier for whatever reason, but that doesn't seem to be the cause here)

The bug gets tracked in https://gitlab.com/postmarketOS/pmaports/-/issues/2299, and a fix is being developed. Workarounds are mentioned in that issue. The download page has been updated to mention this and link to the issue in the meantime.

Is Alpine also becoming immutable, or is this development towards immutable independent/separate from Alpine?

It's a separate effort. 🙂