Return siphon. As others had said, you can drill a small hole. The other solution is using a check valve pvc
attachment. This will stop water from slowing back down. If you use the check valve, install it down towards the pump. This will only allow minimal water back into the sump if the power goes out.
Second, your pump is to powerful for your overflow. You have to options. Install another overflow, or install a ball valve on the return line above the pump. This ball valve will restrict flow, slowing down the GPH
. Therefor allowing you to equal out your flow.
This should solve both issues.