A few edits have come through today where a second person is asking for additional information on another user's question but is doing it by proposing to include their question in the original question. Is there a way to move their proposed edit to be a comment instead?
You are right that these are not valid edits. I suggest you reject the edit (I assume you can do this if you have enough reputation) and in the custom reason tell the user to post a comment if they have sufficient reputation.
It's another example of bad behaviour resulting from users with low rep. not being able to comment. I don't think there is a better solution - you have to have some reward for gaining reputation and being able to comment is one of them.