I rode Sienna for six months bareback before a horsewoman who lives up the road gave me a fully mounted Bates AP saddle.
I know, right?
I lose my right stirrup when transitioning to canter quite regularly.
I keep my seat but man is it annoying, I have to slow to a walk to regain it.
Seems I have Mongolian influences in my riding style because I love to stand in the stirrups when trotting long distances and cantering on the trail. Dunno why I just find it more comfortable.
In terms of posting/rising to the trot I think you will know when you have got it right (if you don't already). You will be able to feel it.
For me it just suddenly felt natural one day and I thought to myself "Oh so this is how you are meant to do it".