Q#1 answer. Yes.
Q#2 answer and a question. Why do you say they are most likely sterile? Do you have proper habitat for the GSH to spawn on at the proper time of the year? Lack of proper spawning habitat could cause them to re-absorb their eggs. I wouldn't cull any.
Q#3 answer. and a question. Do you still see a snail population? If you still see snails, buy more RES, but this time buy them 3"-6" in length to make sure you are getting RES. YP will predate on GSH, I have caught large YP on a 6" soft plastic jerk bait that had a 3/0 hook in it while fishing for LMB in the pond. As for # of Saugeye to stock, that all depends on the amount of panfish fish you want to be eaten. Could be as low as 4/ac, could be as high as 30/ac depending on the amount of forage fish for them in the pond and how many of those forage fish you want to remove. Hopefully Snipe will chime in.
