Is it possible to play Supreme Commander 2 without ever installing Steam, or is Steam required?

No.

That's "No, Steam is required," right?

Yes.

Don't ask two different questions in the same sentence.

It is not possible to play SupCom2 without Steam.

Steam is required.

Suggestion: welcome Steam with open arms.

You can play supcom2 offline though.

Steam has an offline mode as well.

The only problem I have with steam is that it takes 5 mins to launch a game when youre offline... I dont find steam as a platform so bad for any of my steam games, apart from having to run the program.