From the pictures it kind of looks like your tank is not drilled and you are trying to make a siphon style overflow out of PVC. Am I correct with what I think I see? Can you take a pic from the side or back straight on so we can better see what is going on? If this is a siphon style overflow and a newly setup tank then my main recommendation would be two things. First, find out if your glass is tempered. If it is not the proceed on. Second, drain the tank and drill it. That will save a lot of headaches in the future. Drilling tanks is easy and only takes a small amount of time. You can install a small overflow box on the back that will never fail and look great.
I wouldn't recommend the use of check valves to prevent overlows in your sump. They can and eventually will fail. All it takes is for a small piece of trash to hang up on the inside and not let the valve close. Instead you should install a sump that is large enough to hold enough water so if every fail safe failed you still would not end up with a wet floor. Also, I think you may have already done this, but just in case you haven't you can drill a small hole in the wet side of your return to create a siphon break. Just make sure you do it on the bottom of the pipe so you don't end up with a water jet shooting straight up out of the tank.