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)