"the most concerning detail appears to be the vinyl top … "
What?! I didn't know that vinyl tops came on 1957 cars. I had a '57 Sedan de Ville for 22 years, and had done huge amounts of research on the '57 Cadillacs... never once hearing of a vinyl top. They're that rare.
I just now read that the '56 Seville could be had with one. Live and learn!