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I'd look this up myself but don't have the rulebook with me at the moment.
If a Wave Serpent moves 12", you can disembark. I know the unit cannot move, but can they run, and then use Fleet of Foot?
I hope so, or I played it wrong last night.
No, unfortunately not. The only vehicles you may assault out of (after the vehicle moves) are open-topped vehicles and landraiders.