Dang! Whoever thought a 55 year old guy would have unheard of flexibility! Guess I'll go thru the rest of the day feeling like a young gymnast instead of an old fat fart.
For the OP: Sit on your horse and see if you can move your toes around without moving your thigh. You don't even need the horse. Sit in front of your computer and move your toes +/- 20 deg and see if your thigh twists with it.
However, if this is true: "Turn your toes forward and your knee turns in
", then you can guess why I think toes forward can lead to gripping with the knee, which is normally a fault. That is one of the reasons given by Littauer & the Cavalry manual to leave your toes out some amount - because if you keep your ankles loose, then turning the toe in will eventually press your knee in while pulling your ankle & calf away.
Also for the OP: if you want to know how to ride endurance, then ask endurance riders and/or go to some endurance events and see how they ride. 7 Characteristics of a Great Endurance Horse – September 2011 | Perseverance Endurance Horses
"If I rode with my toes outward I would not be able to use my seat correctly and nor would my heel be light and able to give the aids I require of it.
Odd, isn't it. Many great riders have managed to use their seat correctly while having their toes out 15-45 deg. Now, is their heel "light"? Probably not, since a forward seat puts weight into the heel. Is their heel ready to give cues in a way that would be effective in a dressage competition? Maybe not. So should a dressage rider put their toes out 20-30 degrees? Probably not. But if cowboys and jumpers stay on horses with their toes pointing out, then it is hard to avoid the conclusion that it has nothing to do with the security of your seat.
Dressage is a subset of riding. It values covering the smallest distance with the most effort to the horse. Like ballet, it isn't about efficiency. And like ballet, it isn't wrong - unless you are running a race. So if the OP wants to move into endurance racing, she would probably be better off thinking about how to cover ground efficiently rather than spectacularly.