Beechen, actually, said that he'd gotten Cass wrong, and that he considered his mini to be his "redemption," his chance at fixing his mistake and making it up to the fans. Bedard immediately owned up to the Connor error. Should Beechen have realized he did it wrong and then moved on to keep doing it? Should Bedard have decided "oh well, I made that error, but it's in print now, that's what I'm going to have to go with"?
You're utterly, completely overstating both the need for a commitment to an error in a writer (which is nonexistent) and the effect of stepping away from an error by a writer on the story (which can only be for the *good* of the thing). "Radically changing Nemesis' personality" halfway through the story would be bad, yes. So would abruptly deciding to make the story about the Crazy Adventures of Black Orchid and abandoning the whole Trinity concept. I'm really not seeing what either of those actions has to do with the argument. If Busiek wanted to stop glaringly undercutting his whole premise and kicking suspension of disbelief in the crotch with the portrayal of Diana/Tresser as as significant a relationship as Alfred/Bruce or Clark/Lois, all he has to do is... stop doing that. Give that line to Donna, and have Tresser hang back kind of meaninglessly in that scene the way, y'know, Dick does. Have him kind of tag along after the others collect him and not be the driving force of the "get through to the Trinity" program the way, y'know, Kara does. Have him contribute to the thing the way, y'know, Tommie does. She's a major part of this scene, and she manages to pull it off without any kind of pretending that she's ZOMGsuperimportant! to any of the Trinity.
It's too late to get rid of him, sure. It's not too late to stop highlighting his false connection to Diana with glitter and sparkles just to make it as obvious as possible that the wrongness is still in full force. It's not to late to stop making it worse.