It would be cheaper to just buy a servo reverser.
Yeah, you can't reverse a servo by just messing with the 3 wire harness. Years ago, about 7 years ago, my first hobby grade rc was a emaxx and I rigged up rear steering, but the rear steering was backwards so it caused the truck to crab walk. Not knowing any better I thought, easy fix, just reverse the red and black wires and bam should turn the other way. Needless to say I ruined the servo in seconds.
When you reverse the throttle channel on your radio, are you redoing the throttle calibration on the esc? Anytime you change throttle profiles/settings like neutral/center point, direction you need to redo the throttle calibration.