I would say yes to the detent plunger moving in and out to follow the notches.
That is a neat looking motor. Never touched one, I'd do it proceeding with caution. I have no idea how it comes apart. Candace has done a bunch of Berninas, she'd probably know.
It's super clean though, don't usually see them like that.
Tri-Flow oil will, patience, and time will be the trick on this one.
Joe