Hi again,
1. can't really help you there. Haven't done a build in a very long time. But you should teak a look at the code, since a lot of it are shell scripts. And no, building a dependency is not the same as building a docker image, although there are some similarities.
2. each one (BBB, SL) have their own DBs. I would recommend against setting up your own Redis and/or Postgre DBs since they're included in the setup scripts (SL uses an extra postgre docker image, BBB installs it's own mongoDB).
3. SL polls now and then a/the BBB server/s. It's my experience that, once a meeting has started in a BBB node/server, SL only polls this BBB node to see that everything's ok.
4. GL is a frontend for BBB. BBB is your backend. There are other frontends like b3scale, PILOS, GL, etc. Canvas and Moodle can also work as a frontend but (of course) do much more. GL is specifically designed to be a lightweight frontend.
5. since version 2.6 (or 2.7) BBB includes its own stun/turn server. You can have an external stun/turn server. Whichever path you choose is up to you. Can't really say much about a cluster proxy since I haven't had the need to setup one of those.
(for the community: feel free to correct me if I'm wrong in some points.)
Regards,
J.