is it possible to see the revisions made to a student's submission?
No, the Workshop does not keep the revisions of submitted contents. It would be a nice-to-have feature but it has also non-trivial implications on files storage and backup and restore.
is it possible to allow students to make changes to their essays after peer assessment before I do a final mark?
As you have concluded yourself, the current way is to switch the workshop back to the submission phase.