12V pump on 24V supply
The nameplate say the pump can be run from 6 to 24 V.  Since 24 V is within this spec, yes, what you want to do should work.

Yes, the pump will run faster on 24 V than on 12 V.  It will draw more power, but also pump the water faster.  However, the overall energy spent to pump the same amount of water will still be higher at 24 V than at 12 V, even though the 24 V is on for a shorter time.