"...as far as we know that a noun can serve as an adjective in a sentence" ... NO. Assumptions like this will only make your study of English triply difficult. When a noun functions as an adjective, it's a matter of word order; it has nothing to do with noun-phrases. A noun phrase typically begins with a verb in the gerund form. This way, you can treat an action (and its details) as a "thing". Example: "Walking five hundred miles then five hundred more is difficult but worthwhile if I will be the man who'll fall down at your door." In this example, "walking five hundred miles then five hundred more" is the noun phrase.

I'm sorry to tell you that there is not a fixed rule for this. You will have to memorize these noun-noun compounds.

Sorry, but if you can't use a capital letter, a full stop, or a comma correctly, then how do you expect us to teach you the subtleties of noun uses?