I'd make the following changes:
Move the 2 second delay to
before the crew hops out. That makes it harder to dodge shots with a decrew, and removes the irritating two second delay before you can queue a recrew order when repairing your tanks.
Apply an 8 second stun to any recrewed vehicle: it takes a crew a little time to get into their seats and start everything up.
That stops you hopping in and out in combat: you'll stun your tank.
2) add a timer to the vehicle itself that still makes it a valid target by the AI for 4-5 seconds while still instantly dropping the crew?
This is possible, but the vehicle won't be recrewable until it runs out. You'll also have to stun it to stop it autofiring.