If I remove a reviewer in Manage users,  do I also need to unassign them from any assigned reviews?

No, if you delete the reviewer, they will no longer have any assigned reviews. See Manage users and Assigning a submission to a reviewer

How do I see who has completed their reviews?

See The review tables  

I need to complete a review on behalf of a reviewer. How do I do that?

See Editing a review.

I need to assign a large number of submissions to several reviewers. Is there a quick way of doing this?

See Assigning and unassigning a submission to a reviewer

How do I control what data the reviewer can view in the submissions?

See Question tags

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