I think you look like a fabulous athelete in your sports bra ;)
Definitely work on relaxing into your heel. As you said, your knee may be a little grippy. Overall, I think your position is good and has potential to be awesome.
Excel looks really good in the flat pictures. He looks like he is using himself properly, so good job with that. He would make an awesome hunter, but I'm with you, eventing is so much more fun ;) He looks very capable of eventing as well, so really its just up to you.
Oh, and your release is definitely big enough. You don't have to go that big lol, but I guess its better too much than too little.
Way to go!
Every great dream begins with a dreamer. Always remember, you have within you the strength, the patience, and the passion to reach for the stars to change the world. ~Harriet Tubman