If turned off, turning itself on shortly before the next alarm rings.

You could use Syncthing for local folder syncing between devices. It’s been really reliable for me.

My SO’s wifi is called “Connection not possible”

The siblings could run
tailscale ping nodeIP/nodeName
This not only shows if they are connected via a DERP (relay) but also retries to connect them directly a few times.
If that doesn’t work out it can sadly be quite hard figuring out why a DERP is used instead of a direct connection.

I second that. Ask 5 people, get 10 answers and the others are obviously wrong.
Also everyone should use Linux and everything can be done via GUI, but I obviously use terminal for everything so i can’t tell you how to fix it in GUI. Oh, sorry, you need to know how to use the terminal for that specific error you are having.