This sounds like either the drain from the overflow has a problem or the return pump is just to large. Yu need to use the largest drain tube from the overflow to the sump that the bulkhead can handle. If it's a 1" bulkhead for the drain line you want to have 1" tubing to drain into the sump and it should be as unobstructed as possible.
If the drain is setup properly then the return pump may just be to large. That could overwhelm your overflow. That's why people are recommending you divert some of the return flow back into the sump.
The leak is a serious issue. Don't try to fix it with flex seal or anything else. Just replace the bulkhead or if the system is new call the manufacturer and ask for a replacement if slight hand tightening doesn't stop the leak. If the bulkhead is threaded you may have just forgot to use Teflon tape at the connection. If it's a slip fitting you need to use aquarium safe PVC
sealant to connect the pipe.