That's a pretty detailed analysis, nonetheless! That's the kind of info we need!
Off-hand, it seems that if we put a diode in there, it might create a condition where the pump will simply not start, dunno:shrug:
I think that the direction of rotation depends on the starting position of the motor, and that's a 50-50 chance. The only way to force a direction would be to put the impeller in the proper spin before turning it on, but that's not practical, now is it
(I'm picturing an old Ford Model T, with a crank starter)