The second one the focus point is on the player behind the ball carrier. I like the last two, good action. It's tough shooting at 6400 as I find the images go mushy and don't always look sharp, I add a bit of grain(noise) to images like this, I find in a lot of cases they look sharper. Watch the background horizon line and try and keep it as straight as you can.
I know guys that shoot low all the time and it does create a different angle, what it also does is not give you any room scramble if the play suddenly comes your way. I've shot both kneeling and standing, some places don't allow standing. Where to position yourself all depends on what players you're planning on shooting. I've found for linemen(which are tougher to shoot), standing near the line will get you better opportunities for pass blocking or if the guard is pulling around the corner on a running play. Receivers can be shot from almost anywhere, you never know where the ball is going, and in a lot of cases it just comes down to luck.
I look forward to seeing more, thanks for posting these.