Nudges open defense for half a second: hardly possible to react on it, nudges are better exploited by the nudger or simply to combo break.
Disagree. Depends on the player you play with. If I volountarily move into facehug position with a pike, there's a reason for it. Good players understand what I'm about to do, and will prepare their attack for the incoming nudge. The nudge lasts long enough to be a very helpful tool, while not being considered overpowered.
I agree with your subtext that having a second attack direction for very long polearm would buff them a lot, and the current status quo is OK.
BUT! If another thing could be traded in exchange for overhead stab for long polearms, it would add a great deal of depth to the game for polearmers, and look cool too. Just need to find the balancing factor.
At the moment we have 4D, 2D and 1D polearms (could've had some 3D as well, but that's for another discussion). Losing attacking directions is a significant disadvantage, but the weapons still perform very well in certain situations. If pike and long spear were to get overhead stabs, the downside of using one would purely be based on other weapon stats compared to other 2D poles, which in my opinion would be more boring than the balance now.