It's an overflow, it doesn't work on suction. The piping is below the water-line so it doesn't matter that the top is open.
In theory that will work, but I think you need some more research into the specifics as far as flow rates and contingencies/redundancies.
as long as the return flow is less than the overflow and as long as the sump tank have enough spare space to hold all overflowed water during power failure, its good. and at least 1 redundant system is a good idea in case you have overflow blockage