What about simply password-protecting your admin folder, (which you should be doing anyway!) and then creating extra usernames for these additional admins? I would think if you trust someone enough to "allow" them to help you with your website, this would not be a big deal? You can create separate usernames for your protected directory and be able to monitor activity/access via the logfiles that way. Obviously, all users would have the "master password" for the admin login, but they also would have to "login" to the protected directory, so you could in effect "lock them out" at some point by halting access to the protected directory.

If someone decides to abuse your trust, they could still cause trouble as a "moderator" if they really wanted to. I think this "moderator" idea has merit, but it all comes down to TRUST. If you hand the keys to an employee, they can clean out your store on the night shift, know what I mean? If you suspected they were stealing from you, you would take the key away, correct?
So make a "key" by creating a username in your cpanel for your server and they will feel as if they have a unique admin account. You can take this away if they abuse your trust. Just keep daily site backups and you can fix anything they might do that is not "nice". The real key, at least to me anyway, is to make sure you fully trust anyone that you would give access to, above all else.