Jason Todd being featured required an implausible and never explained personality volte face
What do you refer to? Jason's always been about helping people he perceives as innocent and punishing culprits. That he would get involved when a Monitor killed a criminal Jason himself had no intention of murdering fits with his character.
Jason's character was on the whole not too horribly handled until he decided to drop everything, which might be justified by the fact that he'd seen alternate Bruce get killed before his eyes. The exception to the okay treatment he received was that he tended to be the one who didn't have issues about leaving the people of the worlds they were travelling dead or in deep trouble, which had more or less been introduced already in NW:YO. I think it's entirely the wrong note for his character and lazy writing, though.