Maybe they are too long in the back because you sit too upright. Maybe that's one of the differences between a mountain biker jersey and a road biker jersey. Road biker more aero so longer in back. Just guessing though. They look pretty much the same in the pictures. Never looked up why they classify them as different.
And for shorts, well, we don't want to see your butt crack, so yeah, I'm in favor of high waisted shorts if you aren't going to wear bibs.