thanks for all your answers, it's really interesting.
5. Jack played the guitar five months at the Music Conservatory.
1st-2-c: Jack has played the guitar longer than five months.
In this example, it seems clear that "longer" must be the head of the adverbial phrase, since "longer than five" can't modify "months" like "more than five" can.
I agree. "longer" is a standalone adverb, while "more" is not.
I also agree with 'longer' as the head of an adverb phrase. However, in 1st-2b to me 'more' also seems like the head of an adverb phrase.
I've got another example:
6. Sue is not only the best student, but also one of the loudest.
--> Are "the best student" and "one of the loudest" two single NPs or can "not only the best student, but also one of the loudest" be regarded as one NP with ", but also one of the loudest" as a postmodifier?
Looking forward to you answers!