A wet vacuum pump is not nearly as delicate as a dry one and should suffer no ills by being turned backwards.
That said I still would turn the engine in the proper direction.
Reasons for turning in the normal direction of rotation include being kind to the generator brushes and pumping oil in the proper direction. I have always felt that priming a small engine by pulling it through helps to float the crankshaft and give longevity to the main bearings.
Anytime you touch the prop you should treat it as it will fire. Kind of like handling a loaded gun.
Cupping the prop without wrapping you fingers around it is most certainly the correct method.Also use a one foot forward and the other aft stance to minimize the danger of losing balance and falling into the propeller arc.
I have seen many people clear out small engines on Cubs and the like by opening the throttle and turning them backwards. I suppose this works but the same result can be achieved by turning it forward with a qualified operator at the controls.
Some time ago while operating DC-4s with R-2000 radial engines I knew one captain who said to never move the prop after shutdown because it would break the seal of the piston rings and let oil flow into the bottom cylinders.I do not know if this was a valid theory. He was the only one who subscribed to it but it may be worth looking into.
All that said this is really esoterica and the bottom line is if you move the prop by hand be careful and use the proper tecnique.
Blue Skies, Stearperson