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

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. 🙂