If you only need a bare minimum, and don’t plan to heavily use CI/CD, container/packages registries, integrations with other tools i would go with gitea/forgejo (you can always use external tools on top), else i would use selfhosted Gitlab, it has pretty much all the things you could imagine to need with software development and deployment

It doesn’t matter that website loads javascript code for logged in user, as you need a token (which server will give you after a successful login) to authenticate to apis, it is pretty common to do that way

There wasn’t a client side API, but the API was missing crucial validation of user input (eg only checking the mac address but didn’t check who is actually authenticated)

you can play on multiplayer just fine, as long as server have enabled offline-mode

For minecraft you can just use a offline mode in pretty much every 3rd party launcher, like SKLauncher