Usually it's p2p, there's a moment while initiating the connection where some server is needed for STUN/TURN (i.e. NAT traversal).
I assume some service may have a fallback for server broadcast instead of p2p, but none that I know of do such a thing.
Logging in doesn't work - returns 404 record not found. Resetting password does log you in though,
Of course check the implementation you end up using if it truly is p2p. Also you can analyze the bandwidth of services on your server, which may be useful in your case.
That's a good question. Kinoite does not version some directories, such as your home directory or /etc. Rebasing and then reverting the rebase will create some clutter. What I'd suggest is to use some utility to version your files, perform a backup before doing the rebase or snapshot your home directory using the builtin snapshot functionality of btrfs. Maybe something like restic would work for this case.