I suppose it is possible to do multiple chain rings but to me pedaling is a last resort so why bother. I have ridden multi speed bikes a lot and I rarely ever used the front derailleur.
As far as chain tension goes, the spacing between the output shaft on the motor and the jackshaft is set using the correct number of links on the 415 chain. The spacing on the jackshaft to chainring is done with the same method but is adjustable by sliding the motor up or down. One thing I did not show is that I have put an adjustable stop under the rear motor mount to keep the motor from slipping. The torque now tries to pull the motor down instead of back so it will slip and will loosen the chain.