O M Gosh, I remember every lesson when my instructor had us pull the stirrup leathers, cross them over and in front and we posted without stirrups, 3x on each rein. It made a WORLD of difference when we jumped.
Btw, you pull the stirrup leather buckle out to about 1/2way down, then cross the stirrup and leather over in front of the pommel.
I was just going to say that the best way to improve your seat is toride without stirrups, let your leg hang so your heel is under your shoulder as this is where it should be all the time. You seem to be pushing your heels down too hard,they should be light and springy. I think a really straight cut saddle would be better,personally I wouldn't be able to ride well in either of those as they are very forward cut. You haven't developed any contact yet but I would say that was a good thing until you are balanced in your seat with your heel, hip shoulder in line and then you can develop independant hands.