Hey, thanks for engaging.
1. Every normative claim will presuppose a metaethical commitment. That doesn't make the argument less sound. I could say, 'Killing for fun is wrong', and of course, it also presupposes a universal morality. But that doesn't mean that the argument is less sound.
2. This is a plausible objection to Benatar's thesis. It's part of the non-identity problem: you can't really 'harm' a person who doesn't exist. But my argument is a little different from this.
Finally, yes, a strong objection to the argument from justice would be to deny the analogy.