I can give you a short sweet and definite answer. No the rear seats are not interchangeable.
Try this trick. Go to car-part.com and key in your model and year information. Look at the different listings that come up. If the two were interchangeable, the same list of cars would come up.
Not wanting to start a flame or anything like that but I do own a 2000 coupe and 2003 convertible. The coupe's rear seats fold down where the convertible's do not (the convertible's seat backs are one piece).
The convertibles back seat is 6" shorter than the coupe's. The convertible's seat back is not as tall as the coupe's. The convertible also has the plastic clip to hold the convertible cover.
Will they fit? My gut says no (unless you work for TV show).
However, I can see why the OP is confused as within the space of 2 hours to get two very different answers.
But perhaps there is something that I don't understand. Esp since I have never actually attempted the swap. So I will bow out to more knowledgable Mustang owners take over.
I have a convertible and a set of rears seats from a couple...there is no way on God's green Earth that the bottom section from a coupe will fit in the 'vert, (al least not without the use of a saw-all) it is at least a foot too wide
it may be possible to wedge the back, folding parts of the couple seats in. But I haven't taken it apart to try...I suspect even if they fit mounting them would be a PITA