[dhalucario]

Are custom client's allowed? How can I be sure this won't be sold out of nowhere?

[Firmwarrior]

The backend is AGPL-licensed, so worst case you could just selfhost a forked backend and you and your friends could connect to it with a forked client

Open source is friggin great. Some open source game addon manager I was using made a bunch of moves that pissed me off, but it was a breeze to fork it and just maintain my own version with a couple of patches applied