In this kind of situation, it might be best to start by adding a profile message on a couple of the profiles saying something like "What is the source for Benjamin's middle name of Dean, as shown here? As near as I can determine, it does not appear in any of the records, and middle names were very unusual before about 1800. Lacking a source, I intend to remove the middle name within the next few days." The profile message will generate an email to the profile manager(s) and will be visible to any passersby who happen to visit the profile.
What happens next depends on the response you get. If there's an objection, then at least you will have initiated interactive communication with the profile manager. If the profile manager responds favorably, or doesn't respond at all, you can proceed to remove the middle names from those profiles -- and it probably would be OK to remove middle names from other profiles by this same manager without first creating a profile message, as long as your edit summary clearly indicates what you are doing and why. (When you remove a middle name for which there is no valid evidence, do document your action and the reason for it in an edit summary.)