Congrats on being so proactive towards your riding! I'm jealous of the fact that you get to ride all of these horses, and it will help your riding a ton! That said, let me throw in my two cents here. Overall your position is quite solid. (except for on the bay) Everything I'm trying to get at here is a bit nit-picky, but it's just things to focus on for the future.
On most of the horses I'd like to see a bit more of an elastic elbow connection, if you know what I"m trying to get at. That should allow you to have your upper body even quieter, sometimes you get a little "rocking" with the motion of the horse. If you follow with your elbows you can also give more effective half-halts, the subtlest of which would just be stopping the following motion of your hands. Especially on the chestnut horse your elbows tended to get a bit straight. This inhibits the give and take action by locking the joint in an immobile place. To work on this: Focus on letting your upper arm hang loose right next to your body. Your elbows should almost be below your shoulders. You can also ride with a crop in front of your elbows and behind your back, if that makes sense, to get the feel of where your arms need to be.
Perhaps this is just a pet peeve of mine, but when you're not jumping I'm not completely sure what you're doing. Are you riding in two point/half seat, or are you trying to sit in the saddle? I'm not a hunter, so I don't know how you ride around, but as an Eventer there's a clear difference between the two positions. I generally ride around the course in half seat, and then sit in the saddle 3-4 strides out to re-balance my horse. I might just not understand how H/J's ride, but to me it kind of just looks like you're bouncing up and down sometimes. Not all of the time, but a little it here and there.
Reading this section after I've written it, my lack of understanding of H/J might make this completely irrelevant, I'd love it if someone let me know.
Sometimes when you approach the fence you duck very slightly with your shoulders. It's not on every fence, and it's quite a subtle thing, but it is there occasionally. To help remedy this, I love using grids. Grids allow you to not only feel the rhythm of jumps, but focus on your position. I also like to use placing poles 9 ft out from a jump which rides like a bounce. Then you can mentally say to yourself that until you get to that placing pole you are not allowed to think about moving forward for the jump.
As far as your leg goes, it's not bad. The main thing that I would focus on is getting your weight down into your heels. Sometimes you almost get standing up on your toes, and I suspect this is the base of your problems on the bay horse. Really focus on wrapping your legs around the horse. I heard a great skiing analogy once, if you've ever downhill skied you might want to think about this. When you're skiing your ankles are strapped down to the ski. Do you know how those muscles on the back of your calf are always being pulled and can get really sore? That's how your legs should feel when you're riding. All of your weight should sink below the stirrup and not be pushed down on to the stirrup. You should also turn your toes out slightly to allow you to grip with the whole of your leg. I sometimes find it helps to think of clicking my heels together underneath my horse's belly. A lot of two point and no stirrups work will really help you strengthen your core and legs.
Overall your position is quite good, your head upper body in general look lovely. Your releases are adequate, and efficient. You seem to get the horses to good spots, and ride with a nice pace. Good luck this summer and keep up the good work!