It's an amazing episode of TOS. I can't deny that for a second. But when you add a lot of layers upon layers in storytelling over 50 years.
That's why they invented subspace radio in the series. Defeats that whole problem.
It did and it didn't work badly by any stretch. I just can't see how a sustained conflict would go without both sides seeing the other. And the Romulans have been portrayed as tracherous and secretive but honorable. They may destroy their own ships before being captured but maybe I'm forgetting an episode but I can't see them just flat out murdering prisoners of war. It wouldn't seem right. In fact "Yesterday's Enterprise" shows they didn't.
And I can't imagine Earth just letting POWs not be part of the cease fire. Unless the victory by Starfleet is less certain as we've been let on to believe. History, after all, is written by the victors.