For sure the fastback Barracuda is different because the back lays down. 100% correct that the convertibles are different.
This question has came up many times and I don't recall an absolute answer to it.
I think they changed how rear seat back attaches sometime after 69 . I'm thinking one style is held on by screws and the other has some tabs that bend over it. I think with minor mods you can make all 67 through 74 Dart 2 door hard top rear seats work. The barracuda rear seats have a recess in the center top with a fish emblem I'm thinking.
That is correct. On the 72 seat I got, the taps that go behind the bottom cushion do have screw holes in them. Other than that, it fits and attaches the same as the other did.