We can't reply to a comment directly, There must be a reply option under each comments, So that if you want to reply to a comment, you click on that option, rather than commenting again!. (for example: See facebook)
You can @ reply to people. Just type the
@ symbol followed by the first letter(s) of the person you're replying to and their username will appear ready to be clicked. Or you can just type their name after the @ in full.
But regardless, this isn't a discussion forum. It's not a place for comments and discussions, it's for Questions and Answers. Comments are secondary so shouldn't have complex UI and interactions. Just the bare minimum is needed so people can get their query across but without encouraging extended discussion. This isn't Facebook, so we shouldn't try to emulate it.