I'm new to pools but not plumbing. I think you should try what has been suggested for reverse flow. Buy $20 in PVC and maybe some flexible PVC to make it easier to make the joints and plumb your main pump to pull from the returns and push to the main drain. You aren't pushing against 17,000 gallons of water, water pressure works on pump head, or stated similarly, the differential between inlet and outlet pressure. At your pump (with the pump off) the difference between inlet (return) and outlet (drain) is zero psi, because they are both at the same height over the free surface of the water. If running the pump in reverse doesn't fix it, you can call a plumber and have them run a drain snake down the 1-1/2" pipe to your main drain and see what they can push out. Then plumb the pump back in and run again, cycling the pressure on and off with the switch or a valve. If they don't make any progress allow them to run a scope down the drain to see what the real problem is. I strongly suspect that cycling that main drain in reverse is going to free up the clog.
