I read your post several times but I don't understand what the problem you ran into is, if there is one. If you simply want to know the best method to design a UI then you'll receive an opinion-based answer. I believe the best method to design a UI is to generate it on the fly, that is create it by using geometric primitives. If you wanted to change the rounded corner, you'd have to draw it all again. If you were using vector graphics, it would consist into changing some parameter.
Yeah, I'm still not totally sure why we can't just load those items as sprites one on top of the other. According to my partner, Unity isn't able to do that (I could have sworn it could), and we need to use 9slice for "dynamic scaling", but that doesn't make any sense to me. We've got all of the assets created, and they could easily be added to a sprite sheet, so I was just hoping someone could tell me why we should go another route, or why that wouldn't be possible. Thanks!