Thanks for your patience, everyone. Took a look through today, and here's where we stand on grav weapons:
What happens when a vehicle suffers two 6s from a grav weapon? The first hit inflicts one hull point and causes the vehicle to suffer the immobilized result. The second hit does the same thing, and since a further immobilized result causes the loss of an additional hull point, the vehicle loses three hull points in total.
What happens if grav weapons fire at a unit with mixed armour saves? Since the armour save of the target effectively replaces the comparison of strength and toughness on the chart, use the majority save value of the target unit to determine the number you need to roll. Using the value of each individual model, starting with the closest, only applies when taking saves against already allocated wounds, not for determining the number of wounds in the first place, so majority is the way to go here.
Vehicles and cover saves. What's up with that? I know, right? This is pretty much the scarab swarm conga line all over again: they're probably going to FAQ it in a month, but right now, the rules are clear. And the rules were written under the assumption that vehicles only ever suffer glancing or penetrating hits, and specify that cover saves are only taken to negate glancing or penetrating hits. Since grav weapons don't inflict either, the cover save rule ridiculously does not apply. Hide your kids, hide your wave serpents; they're grav-ing everybody out here.