Audiences would flock to a Sci fit war movie with a lot of action, super special effects, and a good story. Star Trek would be happy with a Marvel or Transformers movie level of success.
They'd have to retcon and ignore the fact that humans never saw what Romulans looked like, but as long as the broad strokes are in place the writers are comfortable with changing and ignoring those minor details for sake of a good story. The general audience doesn't know or care about that detail.
I'm not sure they'd have to do that. Just say the Remans were the Romulans' foot soldiers during the war. Although, Spock's dialogue in "Balance of Terror" makes this problematic too since the exact dialogue he uses is "no human, Romulan, or
ally has ever seen the other."
You could arguably get around that by saying the Remans technically aren't the Romulans' allies. They're the Romulans' slave army, and Spock was referring to the Romulans and the founding members of the Federation (i.e., humanity, Vulcans, Tellarites and Andorians). Because you wouldn't call the Jem'Hadar the Founders' "allies." They're the Founders' slaves who serve as servants in the Dominion's armed forces.
Any ground troops Starfleet meets will certainly be armored, and this would extend to headgear. So nobody is ever going to see what the Romulans really look like. They'll have helmets, just like in BOT.
It could be similar to how the series
Space: Above and Beyond did the war with the "Chigs." It's a war with an alien race that wears body armor, but any attempt to forcibly remove their coverings results in a chemical reaction that dissolves the body.
I think it's pretty much implied in "Balance" that it was entirely a space war.
Sisko has a line in DS9's "Homefront" that I've always been intrigued by, since he likens a Jem'Hadar invasion of Earth to a threat "waging the kind of war that Earth hasn't seen since the founding of the Federation." The possible implication being that at some point during the war the Romulans may have launched a direct assault against Earth.
JARESH-INYO: What you're asking me to do is declare martial law.
LEYTON: What I'm asking you to do is let us defend this planet. We don't know what the changelings will do next, but we have to be ready for them. Ben, tell him.
SISKO: Sir, the thought of filling the streets with armed troops is as disturbing to me as it is to you, but not as disturbing as the thought of a Jem'Hadar army landing on Earth without opposition. The Jem'Hadar are the most brutal and efficient soldiers I've ever encountered. They don't care about the conventions of war or protecting civilians. They will not limit themselves to military targets. They'll be waging the kind of war that Earth hasn't seen since the founding of the Federation.