How do I edit Comments.php so I can add Schema markup to comment output?

The comments are outputted via a callback function given as an argument in this function: wp_list_comments().

In your comments.php you will likely find the wp_list_comments() function with often a callback argument like so
wp_list_comments( array ( 'callback' => 'callback_function_name'))

You can copy comments.php into your child-theme and modify the call to the function with a different callback function like so:
wp_list_comments( array ( 'callback' => 'my_comments_html'))

Then find the function from your original callback parameter 'callback_function_name', copy it in your child theme (in functions.php or make another .php file and include it), rename the function to 'my_comments_html' and modify it there with the markup you like.

Hope that helps