OK there are 3 types of user names that form an account in BE.
1) The name of the account in BE - this is just a text name that is a record of how it is held inside the BEDB and has nothing to do with AD itself and in fact does not even have to match whatever you use for point 2 below. (As an example this facility allows you to use 2 different root accounts that have different passwords as even though they are both root, you can give them different names in BE to separate them from each other and then use them with different jobs)
2) The actual username and password of the account needed to do the backup task for the resources) This of course is usually an AD account but may be a local windows account (or Linux Account) depending on what you need to backup. For the SLA and BESA accounts it is usually the same AD user, covering both, and also a Domain Admin. The detail of the configured AD (or local) users are stored inside the BEDB and the used when we need to access the resources in the environment during jobs.
3) The account owner - this is a record of the AD (or local Windows security SAM database ) account that was in use (to access the Windows Server Console and run the BE console) when the BE account was created. The owner is definitely used to handle the restricted account capabilities but may be used elsewhere in the background too. We do not do much testing of what happens when the owner is deleted from AD - hence it is difficult to say what you might experience.