You need a good amount of karma to edit someone else's posts, so that's a non-issue. If it was an issue, the rel=nofollow should be based on the rep of the user creating the link, whether that was the poster or the editor.

You need 2000 rep to do this, which is not so easy to gain, but certainly achievable for a highly determined person. However, what @wookietrader wrote applies -- each edit is marked with the person who has done it which solves the problem.

Anyone can edit. If your rep is low, it has to be approved.

Since you can check what user added what link (using diff), you can still use the reputation threshold. If that does not scale, just use the minimum reputation of the editors' reputations.