I mentioned that I have a physical copy to Trinity (which included Entrenchment and Diplomacy, yay). So that would mean I'm spending $10 on Rebellion alone... but it also means I would never need to deal with this kind of garbage again... The Steam purchase INCLUDES Entrenchment and Diplomacy bundled in, right?
Sorry, I skimmed that last bit.
If you registered Trinity to a different account, or didn't register it but still have the key, you might be able to find it/register it and still get it on the Stardock store.
Rebellion includes all the content from Entrenchment and Diplomacy, yes. Unlike Trinity you can't go back and play just say Entrenchment or Original Sins without Rebellion, but you won't be losing out on any features you have now. And you'll get a lot more like more specialized subfactions, titans, corvettes, game modes, new maps etc.