There is actually a quite quick way to enable Kruber to use his Spear and Shield (And subsequently his Spear) on Grail Knight.
Grail Knights can’t use Hast weapon (Spear, Halberd, Bill, …) but there is 1 that some Grail Knights do use in the Lore, House Bordeleaux is closely tied to the Cult of Manaan and they are permitted the use of it’s symbolic weapon, a Trident
So the simple Loreful fix would be to transform both Spears into Trident and let Grail Knight use them