[Alex van de Wetering]

Me again, Alex. Your worst nightmare.

don't worry...I enjoy this kind of brainstorming very much!

There is indeed a gap between the chain drive case and the hole in the right side of the hull. I know from the crab manual (and I think also from the one Crab in Bovington), that this gap is supposed to be covered with steel blocks/plate, although I am not sure if all Crabs had this, or only later production ones.
The chain box is also fixed on the outside of the hull through springs to an angle iron. This fixation is still present on the Overloon example, so I don't think the gear is pushed inwards. The flange you see in the pictures is a flage to connect to pieces of the chain drive box together, it's not supposed to be flush with the inside of the hull sidewall.
The part of the chain case on the outside of the tank does shows some damage with the rearmost panel bent inwards. I always thought this was a result of the drive shaft snapping, but it could well be that this is indeed damage from projectiles.

Internal ammunition explosions usually result in the turret being blown off or at the very least a catastrophic fire. I see no evidence of either.

Thanks David.....exactly my thought. The story is that Avalon was put out of action by a Panzerfaust that ignited some of the ammunition inside. At least that's the story that is spread on the web and probably also in the museum.