Great to hear that you are nearly sorted.
Concerning machining the air tank plug to take a screw shank push fit connector I have a a spare air tank so can easily rob the plug from that to modify if the matter becomes urgent for anyone. Back burner job for now.
Under seat connectors are known for loosening off and causing odd warning light issues if the seat is moved. I often push my passenger seat right forward fora bit more load space and have learned to check connectors and warning lights before filling the car up! Never a problem checked before loading but both times I got rush headed and didn't check I got the light show.