No they aren't. So they Mandalorians would just bombard the world into submission. And this scenario is if the Jedi had completely abstained from the war.
I don't think the Mandalorians would turn on each other. Mandalore the Ultimate was one of the greatest Mandalorian leaders. He united the clans for a single purpose. They were united in the fact that they wanted a glorious victory. They wouldn't jeapordize that.
We also have to remember that Republic morale, numbers and supplies have run low. By the time the Mandalorians hit the Core Worlds, they have taken control of vital trade routes, gained multiple victories against Republic forces, and just the fact that they were in the Core would give a sense of hopelessness to the people.
Good point, they were highly fanatic. Whatsmore, at this point no doubt the True Sith would have intervened and taken over.