In the errata it explicitly states that the Witch-King must use his own attacks to benefit from the Morgul Blade, but that he may double his attacks if the enemy is knocked prone by the Fell Beast's charge.

May he use the Extra Attack ability afforded by his being Monstrous Cavalry in the same way, for a potential four attacks to which the Morgul Blade can be applied, or is he limited to two for the Blade (assuming no Crown, of course)?

He has 1 base attack, so when charging it becomes to win the fight.
And 4 when the opponent is knocked over.
Sounds the most logical to me.

using mogul blade. 1 for his own attack. 2 for the charge. 4 when knocked pone.

Cheers; was just wondering based on the way it's phrased in the errata.