How can I conduct a code review for an interview candidate on GitHub using a PR/code review in the absence of new commits? Can I use a hack like making a fork, or do I need to clone the repo locally and read through line-by-line? I'm particularly interested in the per-line comments.
You can make comments on pre-existing commits but not the entire repo as one unless it was committed in one commit. The only way comments are usually allowed on Github is if the comment is "related" to a commit, so then Github knows at which point in the repo's history the comment happened.
My suggestion for you would be to make comments on a commit that contains a lot of changes so that you're not going back and forth between many commits.