It won’t flood because you have a check valve in your return line that is working correctly at the moment. If a tiny snail were to be on the edge of the lip of the valve when the pump shuts off, the water will back siphon into the sump until it’s below the level of the bulkhead. But, you never said whether those two bulkhead are within your overflow chamber. If they are, then you are more safe. Because it will only back siphon what you have in the chamber.