To me, the big question is, would Alt-Clark *know* that he risked dooming the canon dimension by going back in time and rescuing Alt-Lois? If he didn't know that, there could be no ethical dilemma.
Besides, I feel that he would choose the concrete action of helping alt-Lois *now* and worry about a nebulous possible future later on.
And another "besides": Besides, he's Superman! He could arrange something else to save the canon dimension, if he had to, and he knew about their problem.
