With modern designs ... the risk is VERY low ... as it appears you experimented with.
For this to really be a problem, you would need to have a single floor drain (which I do) and have the suction isolated to only that drain (which I don't).
In addition to the entrapment risk (admittedly low), having a check valve on the suction line is just another place for a suction air leak to develop.
IMO, except for extreme cases where the pump is many many feet above the pool water level, there should be no need for check valve because the pump should have no problem getting primed and the pump should not lose prime except in the rare times when you open the system (to clean the pump basket or filter).