hey Hotrod, I was thinking the same thing on the horn ring, I go the 57 150 model ring, but I know its not the correct one. The thing on the Cadillac one you suggested is the depth. It seems way deeper than longacre steering wheel, which is at most 2.5" deep.
Another tell is that the rear of the spokes blend into the rear of the ring.
Also if you collect enough images you can tell that the ring actually has two bends in the spokes, the Caddy one you mentioned is one bend/edge down the middle.
That is why I ruled it out on my list.
The one that seems similar is the 60's mopar one mentioned above, but I believe that one has inset grooves.
On the longacre wheel you posted, it seems that looks could be deceiving as the length between the slots and the ring is actually longer than it seems when you cut the foam off.
The is a non coated 15" that looks similar, but its hard to tell if its the same on the coated, you can tell the NUX wheel was previously coated as you can see the slit where the foam get trimmed on the spoke in pictures.
If you do end up getting one please update, I feel bad I insisted that it was a 17" model, but also info all over this board is haphazard and double and triple checking of images should be done.
As far as the mount piece, I have a search for taillights, bezels,trim and parking lights. It will eventually turn up. I imagine is a part for one of the cars used in the movie.