I’m trying to visualize what you’re talking about.
Does the master valve control the pump relay or the injection system?
How does the injection system work?
Why can’t the drip get the chemical?
Can you rewire the controller so that the four valves that need the rid-a-rust activate first and by the time the drip zones activate the chemical is depleted?
What kind of backflow device is on the supply after the well head?