You stated Endgegner was not used lightly in Germany when it was. You seemed to think Wunderwaffe was used lightly in English when it was not. And searching for Endgegner and Endlösung found our comments and a few opinions they sound similar. No evidence or claims of origin. I conclude Endgegner does not originate from Endlösung probably.
Endgegner originates from Endlösung and while it indeed often is used without thought, the question if it should be used lightly.