A question for you about the Motorized Bottom Bracket, does it have a freewheel function or must the pedals turn whenever the motor is driving?
I can't tell you for sure, but I would be very surprised if optibike
doesn't allow you to not-pedal while the motor is working.
In looking at the optibike pictures (optibike uses a system sort of like my bike) I don't think there's any reasonable way to use multiple chainrings. Keep in mind, though, that if you aren't asking the bike to haul 500+ pounds, the motor should be more than capable of climbing any hill you can find.