1. How wide is the soffit?

The new soffit will run the entire length of the kitchen, 11 feet.
As I've said, because of a duct, it will be 12" high (from bottom of
the soffit to the ceiling)

2. You don't *have* to have 18" from countertop to upper cabinets. In
fact, I find that excessive. I hung mine at 16" and in the past have
used 14". The lower they are the more useable they are.

That's what I was thinking of. I don't know how critical that 18"
rule is. It's just what
I've read as the standard

3. I wouldn't use the soffit as the "liner upper" - I'd hang the
cabinets so they are horizontal. That may result in them being
parallel to the soffits or not so I'd also hang them so there is space
between soffit and cabinet. I cover that space with a molding which
is why I asked how wide your soffits are...gotta have space for the
molding.

4. What do full overlay doors have to do with using a crown molding?
Are you saying there isn't enough exposed space on the top of the face
frame for molding?

Full overlay cabinets are designed so the door covers the entire frame
of the cabinet. Which means if you look at a cabinet sideways, the top
of the door and the top of the frame are almost even. Putting on
molding is a little more involved.