You raise many excellent points and valid concerns, but there's an underlying assumption that you don't quite challenge enough.

That is, that Twitter is a meaningful venue through which the advantages of free speech can be conveyed.

If you try to make any of the arguments in favor of free speech go through, but in place of the conception of speech used for the argument, you insert the reality of Twitter, to my eyes the arguments all fall flat.

We should still be worried about government intrusion into and suppression of speech. Even if there's nothing to save, the cure to perceived ills can be worse than the disease. But the part that assumes Twitter is worth anything vis a vis the philosophical justifications for free speech needs more work, I think.