Mike, if you haven't already done so, it would be a good idea to send your post directly to the CC staff. They are few, and they don't come to the forum regularly.
I found your post very interesting, and what I wonder is, what effect it would have if the wireframe-theme.css were disabled. I've seen that SD uses a lot of code e.g. when inserting an image (much more that I would have done manually in the HTML Editor), and because of that, it happens that classes given to the image can't be targeted by the css.
I also agree that some windows in the app are too tiny, I'm especially thinking of the code window where you place custom html. It ought to pop out and become larger.
We used to have the 'placeholder' as an attribute, but it was removed at a time when Chrome did not support it. I think it can safely be put back now.
I don't have your developer background, but 23 years of making websites, experience with DW pre Adobe, 14 years of Coffeecup apps, and about 5 years of using their responsive apps, here's my tuppence worth.