Carve down a wood block against the inverter,
to the connector separation point,
& use it to leverage off the plug (w a wide screwdriver)
(while pushing the button in)
I actually had the same issue in reverse last weekend when replacing the inverter unit in my gen 2. It was difficult to get off but not as bad as yours, but getting it back on was almost impossible to get it to click all the way on. I thought perhaps the connector on the replacement unit had a problem, but wrapping a set of slip joint pliers around it and pulling them forward hard did the trick.