OK let's re-cap what the problem is. The problem is not related to a check valve, shut off valve or any other valve or how high the pump will pump.
At first I thought perhaps the pump pit did not have a vent, as this pit never sees a lid, that would not be a problem. Venting of the main line may be a problem as there are some requirments on pipe size and number of units being served, but the first requirment is that the discharge pipe enters the top of a horizontal pipe and with a "Y" fitting, the "Y" will have the water going the right direction. As this is hooked up with what looks like a "T" fitting, the water runs into the other side of the pipe and splashes in both directions with enough turbulence to plug the pipe.
After that is solved and then more venting still might be needed.