Also, I think Hunter doesn't need to sneak into the Library to get his fix; there are plenty of private spaces in the residence for him to do this. Also, I would think that if Hunter is desperate for a fix, then he's already jonesing from a prior hit and the staff would probably keep him away from the Oval Office in such a state of relapse.
For these reasons, I think it's a West Wing staffer who's crucial to the Biden administration operations. The Library would make much more sense for a quick getaway (for research) for someone who needs it to sneak away for a quick hit.
-PJ
Occam's Razor: The simplest explanation is the most likely one. Therefore.... Hunter.