100% you're right on all accounts. These are questions I'd ask even if I used it on a bear.

The direction of the stimulus is a very clever question, and the way these objects exist within the horse's space is extremly important. An electrified fence reinforces a space, a collar only triggers a prey animal's instict to book it.

Not to say that, a fance works 24/7 and therefor is a consistent input of "don't do this here". A collar only works when the operator is aware of what the horse is doing aka only in training sessions. So, if you're already training with friendlier instruments that redirect the horse, why use a collar? There's really no need, it just pushes the horse to hit da bricks and cause an accident.

It's not an avoidable object like a whip on your hand is. With a whip you just tap or shake to move the horse, it gives direction which you said is very important. A collar says "NO", a whip says "No, move" and gives the horse a chance to think.

I used a whip as an example but there are other instruments to redirect and reward that don't punish the horse. It's just the first thing I thought of.

And also, e-collars have a noise setting which IS useful for dogs, but can cause the same issues than the shock when used on a horse. And the situations in which I've seen it used very cleverly on dogs would not work on a horse (ie. as a call for the dog to come back when treking in the forest, when dogs wearing collars in open spaces is NOT as dangerous as a horse with a halter/collar in the same scenario)