Requesting and tracking changes in collaborative work in Ai

As internal customers of our Ai designer, I and a couple of others need to see his drafts, ask for particular changes and make corrections to, e.g., text, spelling, object placement, and so on, and then later verify that the changes are made. Today, we often request those changes by highlighting areas of the illustration and adding comment balloons, such as "change text from X to Y" or "delete" or "move to below the box."  This method doesn't work very well, at least partly because our designer can't simultaneously see the comment and the illustration he needs to work in, and even if he could, all we can do to ensure changes are actually made as requested is compare the finished files to earlier versions with comments. Of course, this unnecessarily proliferates file versions, too. 

We can't be the only team who has to collaborate in this manner, and most of you probably do it better than we do. What's the best way to request changes and make sure they're made in Ai? Is there some plug-in or other tool?