Mm, fair enough. Overall the argument for "grand theft mammoth will surely be a cakewalk in both the execution and the aftermath" still feels like a manifestation of the thread's tendency to sometimes trivialize opposition Mathilde might face, but I'll give you that one at least. Still not keen on generating more drama in the home stretch regardless, tho.
Look, I was one of the biggest antagonists of doing anything with Morghur, because he is scary as all hell. I'm not one to risk everything for the memes.
But first, this isn't a chaos Warband, this is a chaos aligned village, and that's not nearly as scary, especially when we almost certainly don't have to run through the village, but are just running away. The village calculation was for a worst case scenario. The illusion on top of the mammoth with the ride spell from the Ambers probably covers our problem completely. Basically, this isn't nearly as scary as raiding a Skaven lair when they are next door to Eshin.
The second reason is that a Mammoth is a huge, battle changing mount (not even necessarily for us, it's a strategic resource for the Colleges). Basically, a battle altar, if not destroyed/neutralized, should win you the battle (remember, battle magic is the type of thing that shifts battles, and a battle altar allows for much more than just reliable battle magic, but some real heavy duty bullshit.
The problem with a battle altar is that exception:
If not destoyed/neutralized. Battle altars face the same problems that all artillery traditionally have faced.
First, they are immobile, or at best, immobile while operational. A bouncing cart is not where one wants to carefully cast. From what I can tell, elephants are remarkably steady to ride, even at a swift pace, because of their unusual gait, where they never actually run (defined as having no point of contact with the earth at some point in the gait). Instead,
their center of mass stays at a constant height. Is this perfect? No, but it is a lot better, and can likely be made better with dwarven help with suspension.
Second, like artillery, they are very vulnerable to melee, either from cavalry or by infantry, and thus need to be defended themselves, meaning that they must operate from range, while also being defended by troops that might be needed elsewhere. In contrast, a Mammoth defends itself.
A mammoth with a battle altar is basically the tank of the warhammer world. It's mobile artillery that's infantry destroying. This allows for it to be used to spearhead charges against enemy lines, where the mammoth breaks the lines, then the cavalry charging with it destroy the rest. It's a completely huge deal.