Because a Wraithknight is a Jump-Gargantuan-Monstrous-Creature, can it move 12" in the Movement Phase and then Re-Roll its charge distance?
I can't see why not...althought as an Eldar player, i've never done this (never thought of it) I guess it isn't using the Jump pack in the movement phase..
Does it subtract two inches from its charges?
Nope...
GC's,
MC's have move through cover, which means they are not slowed by terrain when charing
Does it swing at Initiative 1 if it land in Difficult Terrain? If it jumps over Difficult Terrain?
I believe so. Move through Cover states that the charge range is not affected, but makes no mention of this.
Must it ever roll a Dangerous Terrain Test?
Again, Move through Cover has this sorted, no need to roll.