I think the pressure at the check valve will be based on the distance from the top of the tee to the water surface and come from the pipe side, not the pond side. The amount of the draw line that is underwater does not matter. My gut feel is that the 5 PSI max on the flapper check valve should be fine. I would be more worried that the siphon might not pull open the flapper, but I am guessing it will work fine too.