Diplomacy is built on Entrenchment which is built on the Original so you get all the features with Diplomacy, not sure as to what the deal with the Trinity thing is

If you are just starting out though, you may consider starting with the vanilla version of sins.  it keeps out the advanced features and techs of entrenchment and diplomacy.

Starting with Entrechment is ok, or vanilla. If you go vanilla you get a good grip on what the ships quick since that all you really have. Entrechment adds the defenses which you will never go back to vanilla once you play with them! It does add another tech tree so its best to get the basics of the fleet down before adding defenses. Though its not to hard to jump into Entrechement.

Diplomacy though, I would wait till you get the basics down in Entrechment. The AI is a bit tougher and trying to manage the diplomacy side of things (Envoys and tech) can overwhelm a new player IMHO.

Or you can throw yourself in the malestrom of Diplomacy and learn the painful way....well more painful than learning in Entrechment and Vanilla. I can remember my first few game were brutal against the Easy AI. Stupid Advent....

You're all correct!

When Trinity was first a 'thing', it was a bundle of vanilla, plus both expansions.  You would get a registraton for each.  Once the disc version came around, it changed to being one registration.  With Trinity, you get the exact same content, you're still able to play, for instance, Entrenchment without Diplomacy, and any multiplayer action will be compatible with the non-Trinity versions of the game.  When there's an update for one, there will be a corresponding update for the other.  It's the same files, even, except for stuff like the launcher.

Hope that clears things up, and happy gaming!