I have a dead user account that is within ttaad2100m000 (Maintain User Data) which does not have an live unix or oracle account that I need to delete,

I have found that this user is linked to the DEM owner (tgbrg300.user) within session tgbrg4100m000

What are the ramifications of deleting this user? are there other places it could be found within DEM and what is the process for changing it to another 'live' user account

It would be recommended to change the Owner of the Project Model just to be on the safe side. Is this user the Owner of the Version as well?

It is would always be best to transfer the ownership to another user rather then just deleting the user itself.