Overhead stab speed bonus.

[Narvad]

One thing you need to take into account is the range of a weapon. The damage dealing Part of a polearm is the front (for overhead attacks). The weapon can only get to a maximum of damage if its range is used to an almost full potential.
Not doing so results, as you stated above, in a negative speed bonus or in other words in a reduction of damage outcome.
That given, I can say that it's most likely intended (point b) so especially a matter of timing.

For me it's basically the same as point ")a".
Thinking logically, there should be ~20-40% speed bonus from moving forward to target when striking, which does not exist.
Unless game still "thinks" that we're using swing animation, instead of stab one.

Also, if you for example use a PoK or Jormungandr or any other polearm with (overhead) stab mode, you'll see that it's more efficient to use behind cades instead of in direct combat.

Depends on the user of weapon.
Especially with Jormungandr. It has less visible length, comparing to sarissa(which is shorter by stats, not by game model), so it's easier to use, atleast mentally.