Possessive case denotes:
°possession : Peter's book/a cat's whiskers
°authorship : Bacon's essays
° Relationship : a wolf's den; a girls'
In addition, we have to remember the concept of compound word, proper noun, common noun and by using possessive case who gains predominance.
Now coming to the examples:
Peter's book — no article(proper noun)
A cat's whiskers — article (common noun)
A women's magazine— article (compound word
A wolves' den, a girls' school are compound nouns in addition to being in possessive case; they refer to neither of the two words but to new words( new concept).Let they be viewed in this light.
But in an ' interesting magazine' or a 'prestigious school'— these are mere nouns with adjectivals.
GET RID OF HEAD WORD CONCEPT. THIS IS AT THE ROOT OF ALL THE TROUBLE.
This rejoinder is by way of an explanation of my submission above.
At the outset we like to mention the most important and most comprehensive principle of English syntax : words relate according to sense. This is at the core of all the rules of construction including that of use of articles before possessive.
All determiners (articles/possessives/numerics/demonstratives etc.) are essentially adjectives. This may sound over simplification but it is actually so. We name them differently because their functions are different.
Articles relate to the noun which they limit. It is an index pointing to that noun. An article precedes the noun and when an adjective (here possessive) precedes the noun, the article goes before the possessive and the article's power of limitation may extend over the possessive as well. Article together with the possessive becomes the governing word of the noun governed.
As a general rule before nouns in possessive case we don't use "the" if the noun does not take "the" under normal situation and we must remember that article " the " agrees with nouns in either number. Equally important is to remember that articles are used in the same way before possessive as before any noun that don't have an 's. However, the conjoined concept of article along with possessive may not be lost sight of.
To sum up, possessive nouns are also determiners; like articles they mark nouns; when they are together, their togetherness impacts the noun governed; possessive nouns differ from other determiners in the sense that they themselves often require determiners(articles are one as such). The examples are not analyzed again to avoid repetitions.