I like the idea of having a fallback theme. It sort of lets you play around with creating child themes with Cornerstone which I think is a great idea; sort of like how the current OpenCart default theme works.
Um, I don't usually include the payment or mail folders in my templates though, as I let those fall back to OpenCart's default. I don't think that will create too big an issue as it appears you've just copy/pasted them from the original default theme? I'm assuming theres not a way to have ANOTHER fallback to the original default theme. So you have CS_Child_Theme which falls back to Cornerstone_Default, and if anythings missing in Cornerstone_Default, it falls back to Default. Maybe that's a lot of unnecessary-ness though as long as the Cornerstone Default theme has those files correct.
I'm thinking there are only a few people interested in developing OpenCart theme's in HTML5 right now. How many that would be interested in it and just don't ever post on these forums, I couldn't tell you. I've only just started dabbling in it, but have yet to use it for a real client project. I *think* the majority of OC developers feel the same way. (not that I'm a mindreader or anything and that being said, I'd still be interested in it!) Maybe it would be a nice project when 1.5 comes out.
Also, understandable about the tables and everything. First beta and all!