Selfhosted

First of all I agree with most of your a, b and c points, just would like to point out that while it's true that Docker containers provide an extra level of security they're not as closed down as people sometimes believe, but as a general rule I agree with everything you said.

But you're wrong about the way Plex works, this is a quote from their documentation:

So, your Plex Media Server basically “relays” the media stream through our server so that your app can access it since the app can’t connect with your server directly.

If that's not clear enough:

Your security and privacy is important to us. When you have enabled secure connections on your Plex Media Server, then your streaming will continue to be secure and encrypted even when using our Relay feature. (When using secure connections, the content is encrypted end-to-end and tunneled through our Relay. The connection is not terminated on our servers and only your Plex Media Server has the certificate.)

So it's very clear data is streaming through their relay server, which goes back to my original point of I expect that to be a paid feature, it's using bandwidth from their relay servers.

As for the security again you're wrong, authentication happens on the Plex remote server, not on your local one, which is why you can't use Plex without internet (part of my dislike for them). So you connect to Plex remote server and authenticate there, you then get a client that's talking to the remote server, even if someone was able to bypass that login they would be inside a Plex owned server, not yours, they would need to then exploit whatever API exists between your home server and that one to jump to your machine, so it's an extra jump needed, again similarly to having Authelia/Authentik in front of Jellyfin.