What if Gandalf the White killed the Witch King?

[Inziladun]

Originally Posted by Moonraker

Having killed their great commander, and part of Sauron's power, who would then have the confidence to take on Gandalf at the main gate? The orcs would flee before his face just like the Uruk-hai did at Helm's Deep. The will of Sauron was broken at this point.

No. Sauron's command and control over his troops actively engaged in battle wasn't affected by the fall of any particular minion, even one as powerful as the Witch-king. At the Morannon, there is mention of the Power that drove Sauron's armies and "filled them with hate and fury". It was only after Sauron's attention was elsewhere that they feared their enemies.

Originally Posted by Moonraker

New leadership would have been necessary to re-inforce the will of Sauron, and I could imagine Sauron ordering the Mouth of Sauron into the fray to restore order in the ranks of the legions of Mordor.

The Mouth doesn't strike me as a warrior sort. More of a cunning counselor/politician who liked for others to do the dirty work.

Originally Posted by Moonraker

I believe the soldiers of Rohan did indeed use crossbows to assault the Mumakil, targeting the eyes of these monsters, and this point is stated in the relevant chapter (Battle of the Pelennor Fields). The obvious thing to do from the high walls of Minas Tirith was to use archers to bring the Mumakil down.

There is no mention of the Rohirrim using crossbows. It was the men of the Morthond Vale who shot at the eyes of the mûmakil, and that by "bowmen". I think the walls of the City would have been too high for any targeting of such small areas, and it was said more than once that Gondor and Rohan didn't have a lot of archers anyway.