texas cowgurl, you alone know if you are bracing or not. It is impossible to tell from a photo.
There is nothing magic about heels down. When it happens, your heels can function to absorb some shock - but only if there is weight in the heels, which means pressure on the stirrup, which means part of your support is coming from the stirrups. It is important if you are jumping, perhaps, but not for western riding.
Apart from that, toes up stretches the calf muscle, so your lower leg has a firm yet flat surface to grip the horse if needed.
It also makes it easier to brace against a sudden stop with support from the lower leg.
"One could easily argue that this is some of the best equitation in the world."
It is dressage. It is excellent dressage, but dressage is not the end all of riding. Dressage emphasizes collected gaits. Western riding, on the whole, does not. A position that is mechanically sound for riding a collected gait is not mechanically sound for riding fast, or trail riding, or jumping, or polo, or many other things.
Her feet were not absorbing shock. They were giving cues, constantly. Outstanding dressage, but not western riding.
There is no ideal amount of heels down. George Morris, writing about a forward seat, says the toes should point out 10-45 degrees, depending on the rider and horse. He's right. No one can say XX deg is the right amount because too much depends on the rider. Heck, my left foot always sticks out further than my right. That is true when I go jogging, as well. It's just the way my body works.
Same with heels. Heels level works fine for many people, including Charlotte Dujardin and my youngest daughter. I ride a western saddle in a forward seat, in part because my western saddle has no swell and is a "slick seat".
On short, thick little (13 hands) Cowboy:
On Mia, using an Australian style saddle:
The best advice I've seen about "position" comes from VS Littauer. He wrote there are only two important tests of your riding position: 1 - Are you in fluid balance with your horse?
A horse doing a collected gait has its balance to the rear. One galloping has it to the front. Going around a tight turn, to the rear. A horse's balance shifts and our balance should normally match the horse's balance (or be slightly behind for stability). You can either move your
balance over the horse's, or teach the horse to move his
balance under you. But your horse will perform best if your center of gravity is close to his. 2 - Can you give the cues you need to give to your horse?
In the dressage video, she was constantly giving cues with her heel. That is part of why that position works for dressage. She could not cue the horse the way she needs to if her feet were forward.
But I use a very simple set of cues, since my horses and I are all simple. I almost never use my heel for a cue. I almost never feel the need. I use my lower leg, but almost never my heel. I'd flunk a simple dressage test, but it works fine for trail riding.
If my horse is getting nervous about something ahead, I slide my legs forward. This tells my horse "Don't try to turn. I want to go forward and you can do it." That is a cue for them. It is not one needed in dressage, but it is one I need with my horses pretty often.
If you are in fluid balance with your horse, and you can give the cues you like and need to give, then your position is fine.
On the whole, texas cowgurl, I "like" your position better than enh817's - but then, I don't ride the horses she rides with the goals she has. Larry Trocha, a guy who teaches cutting and reining, would "like" her position better than mine. I like Larry Trocha and I tried following his advice for about 3-4 months, but gave it up because it didn't match the riding I do. My main riding horse is a 15 hand, 800 lb Arabian/Mustang mix. He just moves differently than a Quarter Horse bred for explosive power. I like Bandit, but he's not built the way a good cutting horse would be:
Think about Littauer's advice. Unless you are training for a specific sport or expecting to be judged, only you and your horse can say if your position is right for what you both do.