OK on the return flow, since when the pump is off the well doesn't fill up quickly then there is no siphoning. If the far end of the dump pipe was underwater at the swale then it would siphon back. A check valve is usually at the pump and I think in some wells there is one at the top also. In your case of a shallow well, one at the top would work just fine. With deep wells the one at the bottom keeps a solid charge of liquid water in the pipe when the pump is off. The reason for the pump in the first place is as you suggested. They keep the external water pressure on the pool from possibly floating the liner if the ground water gets higher than the pool water level. In your case, no liner, it still maintains a positive pressure on the pool walls.
Al