Rather than go through your message piece-by-piece, let me discuss some
general principles from the point of view of an email implementor.
While the distinctions between multipart, application, model, etc. may seem to
have certain esthetic purposes for you, they have very specific purposes in
email. In particular, multipart is very special; it is the fundamental
representation of MIME structure. It shouldn't be overloaded for any other
purpose.
Another factor, that is often overlooked, is that each top-level type has a
definite cost. A case can be made for adding MODEL since it is fundamentally
a different entity; it is neither a presentation object (or rather, is not
necessarily one) nor strictly application data. On the other hand, it is just
as improper to restrict model to a particular, limited class (essentially 3D
images) as it would have been if a VISUAL type was created that specified only
flat, static images.
Subtypes and parameters exist to tighten up a specification. Use them.
I would suggest consolidating your proposal into a single subtype, 3D, and
then use parameters for the futher detail. So you'd be using MODEL/3D, but
there would be space for other kinds of models.