1.) Depending on the team, code review is either done by everyone non-junior or by the people most responsible for code base and expected to know most about it. It is last approve/reject step before admitting code.

So yes, this person will do code review. And code review has more functions then mentoring.

2.) Mentoring starts on task selection. Latest when junior start working on it - with senior occasionally checking the junior out while the task is in progress. Code reviewer might be someone completely else who might have noticed that junior is working on code only after it is done.

Sane mentoring should not start when junior finished the work.