If I had a "sweeping claim" to discuss, how should I have posted it?

See "give more explanations and context". If you're concerned with "never find a practical way", that's an entirely different discussion than "isn't Turing-computable" (in this community, if something has a strictly technical interpretation, that's what is defaulted to). Give enough context so that a reader knows what you're concerned with (practical applications, apparently, see I wasn't aware of that), instead of a somewhat theoretical sounding claim (which you apparently meant in a more practical way) with a proof that turns out to be wrong, given that strictly theoretical claim. Also, I was only pointing out shortcomings of your proof, to do so no stance regarding Turing computability is required. However, there is no reason to assume that superdeterminism would require incomputability, on the contrary, as long as the true determinist laws of physics are computable, the universe would be as well, no?

As an aside, would I in general be expected to read all 300+ comments on a post before commenting?

Well, at least the top level comments with a couple of upvotes, so you don't repeat one of the main responses? That boils it down to 35-ish comments.