Here is much more knowledge what I can offer ever, but…
- None. CDN or external fire storage like S3 are optional
- Yes. You can even limit registration per domains. Or just allow invites.
- Of course, this is not Mastodon
- Discourse is open source so you can check it out or just trust everyone else who aren’t connected to CDCK. But Disourse doesn’t need such 3rd party services that can do such things. But if you use 3rd party things you must be sure those aren’ doing funny things.