I have found KB solution S-028869, which describes the process of aligning PDM users with domain accounts that were either created separately or where domain accounts have been renamed.
The document attached to the solution states "The “Admin” login cannot be renamed or linked to a Windows login – it must remain as is."
So that answers my question as to whether or not the Admin user can be linked- it cannot.
If I decide to repost my ongoing problem with the locked accounts in a different thread I'll note it here. Otherwise, I would still appreciate ideas.
Thanks for reading!