"UV gel cured in a clear negative? Knox gelatin or other? "
I use Knox gelatin since it is easy to obtain and cheep. I like your idea of using ballistic gelatin since it would be clearer, has that worked out well with repeated usage? any degradation of the clarity over time?
"Dremel cut the top ring off for the back plate?"
Yes, that is exactly how I make the back plate. for my back plates I have switched from UV gel over to powder dental acrylic with liquid monomer, The reason I use the dental acrylic for the back plate is that it is not formulated as an adhesive and so it is easy to remove from the smooth surface after it is cured. I find that UV nail gel bonds so strongly to glass or stone (I use a polished marble tile since I broke the hand mirror I was using) that it is difficult to remove without destroying either the back plate or the smooth surface that you are using to cure on.
"Sanded with fine (400-600 grit) sandpaper?"
I start by rough shaping the back plate after it is attached with a stone dremel wheel in order to remove any trace of the joint between the back plate and the shell. Then I use 180 grit sanding sponges that I found at home depot, they are perfect for this application because of the flexibility, you can bend them around any corner or into any shape you need, I find that sandpaper doesn't like to bend into smooth shapes without creating a sharp crease. I picked 180 grit only because that was the finest grain available for the sanding sponge. The resulting surface is smooth to the touch but hazy in appearance, the hazynes is resolved the next 2 steps.
Clear style acrylic nail polish then UV gel over that?
Yes, that is it exactly. It is important to use a translucent nail polish because I have found that they do a good job of hiding any imperfections in the shell and not showing brush strokes after the final UV gel coating is applied. Opaque / solid colors show brush strokes and imperfections to easily after the UV top coat is applied.