Good for you to be trying out for an NCAA riding team...best of luck with that!
As far as critique, there are others with WAAAY more experience (practically EVERYONE HERE), but I'll give it a shot on a couple of things!
First of all, to simply address the issues you brought up...your heels look great, I see them as being perfectly far down! I also don't see you as riding at ALL in a "chair seat" position. I think overall, everything looks quite good, and the only thing I would question is the fact that you are leaning rather forward...although, I get the feeling because that is basically your position in ALL of the photos (except where you appear to be standing still) that the forward leaning position is what is required for proper Hunters?
Again, others would be the better judge of that than I, so definitely wait on better critiques!
I personally see your riding, in terms of the two issues you brought up being concerned about, as looking quite good!
Again, best of luck to you and keep up the hard work!!!
P.s. Your mare is lovely! :0)