Leo Messi, who lives next door, is the greatest football/soccer player in the world.
is "who lives next door" an adjunct or a modifier?
My thinking goes like this. If I isolate the non-restrictive relative clause
who lives next door
my intuition tells me that there is no "head" to be modified, which makes this non-restrictive relative clause an adjunct.
However, if I assume that non-restrictive relative clauses have a "head" and that this "head" is external rather than internal (meaning that the head in my sentence is "Leo Messi"), then the non-restrictive relative clause is a modifier.
My assumption, therefore, is that modifiers are attached to a "head" (i.e. modifiers "modify" a "head").
How do linguists analyse non-restrictive relative clauses?
Thanks in advance.