And this I don't buy in any way. Firstly, while the Federation dolts might allow for specs (which I think Starfleet Command would really balk at) they couldn't thoroughly replicate the specific ship right down to distinctive variations and wear-and-tear marks. Maybe if Kirk were in a drugged state which we can see he's not.It also follows that they insisted on having complete, and I mean COMFUCKINGPLETE, information on this ship and its captain before agreeing to the meeting, and the clueless Federation bureaucrats being the helpful dolts that they are, gave them complete specs on the Enterprise, Kirk's complete medical history, maybe even have Kirk do a little match.com video from his quarters where he expresses just how excited he is to meeting these oh-so-dreamy Gideons.
Replicating just the interior would take up significantly less room. And I agree that the overpopulation needn't be literally shoulder-to-shoulder. But it still all comes down to cheap writing.The bigger reach is how, on a planet where supposedly there is no place one can go that isn't crowded with people, they managed to find the room to fit in an exact replica of a 947' starship, and keep some mischievous locals from sneaking inside for some quality alone time. Even if they left off the nacelles, that's still quite a lot of real estate being taken up with this con.
"The Mark Of Gideon" is really an example of one of TOS' strengths in the face of the periodic faltering: the strength of the cast and the show's general ambience help carry it through what could be intolerable in lesser hands. With most things being equal if this story had been a first season TNG season it likely wouldn't have done as well because at the time we weren't familiar with the TNG characters and they were struggling with getting a handle on their roles. Some of third season TOS feels like first season TNG, but it's the strength of the characters and what had already been established that helped TOS plow through while early TNG was disadvantaged in that respect.