Hey Mike,
Whatever wheels an EV motors from will be the same wheels that EV will regen with. The Spark EV is FWD, and therefore the regen is FWD as regen in EVs is done by their traction motor.
Because the Spark EV is FWD, the rear wheels only feature friction brakes and the e-brake.
I don't have experience with flat towing procedures, but I would reason that because the Spark EV has a direct drive transmission, the wheels would turn everything including the motor and the fixed gear reduction part of the transmission. And that also means that 'N' does not physically do anything to uncouple the wheels from the transmission and simply instructs the motor inverter to 'freewheel' electrically. So it could be fine depending on how it's been designed, or you could be spinning gears dry while the car is off and add unnecessary wear. It's ultimately your call. Still probably worth consulting the manual or corresponding with GM on that to collect more info if you do want to explore flat towing. Without further investigation, putting the wheels up seems to be a reasonable course of action.
If anyone else can chime in on this topic, I can't really say much more without looking further into it.