If you happen to link an external object that was made unselectable in the original blend file, You will have no interactive way to make it selectable again, or local, or even delete it from the current blend file.
The good news is that it can easily be done by a very short Python script:
import bpy obj = bpy.data.objects['the_untouchable_object'] obj.make_local()
It should also be very easy script a loop that will find all those linked objects,
I will post an example when I’ll have a minute to breathe… 😀