Oh ok, the first thing that comes to my mind is Kodi. Depending your pc it might be a bit slow but that should work.

Edit: are you on linux or windows? If windows, you should really consider linux. Old pc comes back to life with linux as it needs way less ressources than windows

I guess you meant something to act as a remote desktop? Something lightweight better than a remote desktop for your use case is to setup a FTP server or a samba share. You will then use some media player client to read from there

I don’t know about jailbreak, but you can sideload games/apps. Will send a link in mp

It already exists for quest (vr head set) which is android base, so I guess we will see one for phone at some point.