To ADD the AD info, Yes Domain admin account is needed.
This is needed to configure the access to AD.
It does not check for user existance. this should exist in AD
in the backgroup it will pass the user credential info to AD ...if exists and password is correct you wil be able to log in ,
and once you have created a user in the SEPM and then log in using this user id it just a lookup that is carried out to AD to verify the user?
You can use either the Active Directory User name in this field, or any other User name desired. For clarity, it is recommended to use the Active Directory name for the AD account you intend to tie to this log in.
This will not check when you create the User; it will pass the log on info to AD..