Just to clarify , the old sitemaps won't disappear from WMT. Sure the files have been removed, but they still sit there in the WMT console, and the only way to seem to remove them is using previous submitters email address' ( we have those ) and passwords ( we don't have those )
... is frustrating and a concern incase Google get's confused between the 2.
Sounds like a business problem of not having good rapport with folks... and/or giving up too much control over accounts and spreading too thin. Take the lumps and go from there. This being a learning moment for anyone else out there who wants to let others in on their accounts.
Sometime's it's unavoidable when DM contractors etc. are involved. I take the point and the good housekeeping benchmark, but this is more of a technical question with regards to Google Analytics workflow and business rules.
Wish I had an answer. Only thing I know is when something is wrong (from the get go) it's twice as hard to fix than if it had never been done. Look at it from Google's side... they have to honor the password paradigm or lose all credibility.
Sounds obvous but... If you have the previous submitter's email address, either redirect the email address to you, then do a password reminder or if he/she gets the emails then ask him/her to reset the password.