6. Couldn't Starfleet then start shooting at the Romulan homeworld from their outposts as a response since they're at a close enough range.
Definitely. And I guess that would always have been an option even in the "big RSE" model. We see the outposts rather sparsely positioned, and no matter what the scale
of this map, this very probably indicates that each outpost has weapons with enough range to close the gap between the outposts. Which gives them enough range to hit the Romulus dot, too, more or less.
May I remind you, that Kirk felt his task to be to first ascertain who had been the attacker and second to avoid a new war with the Romulans if possible.
The Federation doesn't subscribe to a "shoot first, ask questions later" strategy / policy, and I think that's a very good thing.