If I find an edit which caused more harm than help by a new user, and which was subsequently approved, is there any action I should take, in addition to fixing the edit? (And in this case, should I just fix the edit, or first rollback and then fix?)
If it is a one-time thing, just fix it however you like – roll-it-back first or don't, whatever float your boat:
If the edit was too drastic, it is usually easier to roll-it-back first. This changes the status of the edit from "approved" to "rejected", and the editor loses their +2 rep. points – hardly relevant but it sends a message.
If the edit was minor, and rolling-it-back and editing it again feels like a waste of time, go ahead and fix it directly, skipping the rolling-it-back step. The editor gets to keep the +2 rep. points, but no big deal if the edit was in good faith.
On the other hand, if you feel the editor is systematically suggesting poor edits, or that a particular reviewer is systematically approving poor edits, then it's best to talk to the perpetrator directly. Just ping them in the comment section – they'll get notified. Let them know that their behaviour is unacceptable, etc. If possible, let a moderator know, either by raising a flag or in the chatroom. They will not do anything in particular, unless the case is specially serious, but it won't hurt to keep them in the loop anyway.