Point is,
Crashing Cavalry,
SoAM, or the phrasing in general will need some kind of clarification.
If the ruling goes that "characters cannot use skirmish special abilities" = "no skirmish special abilities in their skirmish phases", then that applies to any skirmish special ability. It has to
by definition, regardless of how we think it should work.
SoAM knocks this out of whack. Substitute another phase in there and you'd get "characters cannot use maneuver phase special abilities" = "no maneuver phase abilities in their maneuver phases", which is nonsensical. Either you somehow limit the characters involved, for example with this global/targeted stuff (introducing a new concept to LotR), or Grima shuts down all maneuver phase abilities, which I doubt is what Decipher intended.
Like I said, I think limiting this to the affected card's game text would solve the problem (or just errata Grima as Burke suggested), but that doesn't carry any weight.