Return is (almost) always done by backflow thru the pump- the drain is nec in case of overflow conditions.

Easy to install- thruwall to barbed fitting coupla inches below top o' bucket, polytube returns to res. If level goes high, it'll overflow thru this- if not, when pump turns off flow will reverse thru pump.

You should be able to do all of this w/o any rigid pvc. have a look @ "Ebb-n-Grow" commercial systems. ( They don't use an overflow return- they use a high-level float switch, but you can see feed layout).

Pump flow rate is dependant on head, plumbing drag, intake head. But a 291gph should do ya fine.