Our anti-siphon valve for manual pool fill continues to leak after installation after several months. We have changed it/repaired the leak 4 times in six years. Is the anti-siphon valve required or can I use a straight-line on/off valve to turn on the water to the pool?

The anti-siphon valve is to prevent pulling water from the pool into the water system. So, you need one. If you manually fill a pool and the hose is in the water, that is against code if it is not connected to an anti-siphon valve. I have seen pool filling with a large hose, about 3 ft. above the water, not connected to an anti-siphon valve. I would ask your local jurisdiction for guidance.