I don't imagine there'd be much of that uncertainty, because it's a rare event. It's really hard to pull someone down backwards from behind unless you have a really great grip & are dug in pretty well yourself. If you have such a great grip, who cares how you got it?

Seems like the same way they extended grabbing the face mask to grabbing any helmet opening, horse collar should go similarly. If the object is to protect the neck, what difference does it make what part of the helmet was grabbed? If the object is to protect the knees, what difference does it make how the player was collared?

You would make almost any tackle a potential foul. That is silly on so many levels. It is different to grab something that once grabbed can be jerked in a direction. Not so much the case if you grab the helmet without grabbing onto an appendage directly.

Peace

How do you get that? It would still have to be from behind & in the vicinity of the neck with the hand reaching in behind the neck, & the player's upper body would have to be pulled backward relative to the lower body. It's pretty rare to see that happen! But if you do see it happen, why should the rules make you see what part of the player's equipment was grabbed? It makes no difference as to the danger the provision was trying to stop -- the player was jerked back in that manner -- but then you should have to see whether that hand was just pulling the shirt by the collar or pulling the harness under the shirt?!
How is that different? The rules makers wanted to reduce the danger to the neck once they saw players getting grabbed by the recently-added face mask when the chin strap is on tight. Years later they saw that the player's neck could be jerked the same way if another helmet opening were grabbed, so they amended the rule to treat that similarly.

Here they looked at the effect on the knees of a player's being pulled down by similar equipment attached to the upper body. If it's possible for a player to be so pulled down by some attachment other than the harness, such as a shirt collar, why differentiate between pulling one & the other, especially when it can be hard to see which was pulled? (That was another reason for adding helmet opening: cases where it was hard to tell whether the face mask or another edge was grabbed.)