If they fail their DT test then they take d6 wounds instead of 1. I'm pretty sure that's the only difference for chariots...
Yeh, DT test only when charging, marching, fleeing or pursuiting through DT. I think you could argue that the tank would need to take a DT test from the curse if it moves into a contact of an enemy as it should be counts as charging. Otherwise just ram it with a cowboy. It will take time to bring down the tank but should be doable. Poison should also work the same as it will work against anything else. You just need loads of shots and eventually some armour saves will fail BR Agrem
The best options are... Magic (Searing Doom ) Old Blood or Scar-Vet with a defensive kit and a great weapon Kroxigors