I mounted two battery packs too. I posted pics in the ebikes pictures thread. I weight 240 lbs and it gets me to work and back with pedalling for 30 km (20 miles), sometimes with head winds both ways along the St-Lawrence river. I haven't tried it yet with the batteries connected in parallel yet. I just switched the connector to the second battery before.

I was wondering if a 20 teeth crank would make the pedals turn too fast if the throttle was at maximum.
I wonder if a larger crank should indeed be used to limit the rotations of the pedals to a safe level in case the throttle is too fast at maximum. You'll be able to see this with your testing.

I don't expect freewheeling would be a problem. If the throttle is at off, the motor should not move if the pedals aren't used. Pedalling will turn the motor, but again if the throttle is at off, there should be no force exerted on the pedals. It shouldn't be worse then when the motor turns with the wheel when the motor is off as it is now.