There is no need for the charitable assumption that societal value is a defining consideration (which, for the record, is different from your previous charitable assumption that what promotes mental maturity must have societal value). Because as you said, there could be other qualities such as pleasure that would change Sklar's mind about whether chess should be taught in schools or not. Which would mean that Sklar doesn't consider societal value to
be the defining consideration after all, he just hasn't taken into account other considerations that may or may not be relevant. Unfortunately, we don't know what Sklar is thinking beyond the specific argument that he lays out.
The important thing to remember is that whether societal value is crucial to Sklar's opinions regarding chess or not holds no bearing on the answer to the question, although it does affect the strength of his argument. Is his argument compelling? As we have discovered, not really. The assumption that societal value is critical to deciding whether chess should be taught in schools or not is actually one that Sklar makes, not one that we must make.
Think of it this way: Who cares? Sklar made a faulty argument and he is now permanently unfit to be an attorney, but does that matter to us? Is the fact that Sklar will probably never score above a 140 on the LSAT important to the score that we're trying to get on the LSAT? You're essentially evaluating the strength of Sklar's argument when your job is to discover what his point of disagreement with Talbert is.
At the end of the day, it doesn't matter why Sklar thinks chess should not be taught in schools. The question is regarding what Sklar thinks about chess being taught in schools, which is that it should not be. Sklar explicitly states "my objection to teaching chess to children..." is all that you need to prove that choice (D) is correct. His argument does not follow along the same plane of reasoning as Talbert—one talks about mental maturity and the other talks about societal value. That is why none of the other answer choices can work, because the two are simply articulating different types of reasoning. And that is exactly why the stimulus was designed as such, to trap those who make faulty inferences by attempting to connect two clearly distinct scopes of argumentation.