i agree with kenumdrem. you don't need the border. it detracts from the composition and the edge of your design will be defined by the shirt. a border is redundant.
Beside that i really like look of it and the color scheme.
Nothing i can do about the border now. And as for the back of the shirt, idk how threadless does things, but if they could do the print with just the front shirt and drop the back, I'd be fine with that.
Agree that the border is too much, it takes away from those awesome clouds you have in there. Interesting concept, I think it needs some tweaks though.
About my design