No, and no.
Just because something is a modifier in noun phrase structure does not make it into an adjective. It's just a modifier.
From the Oxford Dictionary of English Grammar p254:
Modification is a general term. Nouns are typically modified by
adjectives (strictly speaking, adjective phrases, e.g. lovely weather),
prepositional phrases (e.g. the food in the fridge), or relative clauses (e.g. the house that was demolished); adjectives and adverbs
are modified by adverbs (strictly speaking, adverb phrases, e.g. much
warmer, very warmly); and so on.
And futher, on the subject of 'adjectival', p8:
adjectival (n. & adj.) Loosely, (a word, phrase, or clause) behaving
like an adjective (including single-word adjectives); e.g. in a damp
cloth, the word damp is an adjectival element.
The term is also used for examples like the following:
the greenhouse effect
the man in the white suit
an I’m-all-right-Jack attitude
Some writers informally use the word adjectival to describe all of the
italicized strings (or even say that they are adjectives), but this is
infelicitous, since form and function are being confused: the first
two examples involve nouns as modifiers; the third example involves a
prepositional phrase; and the final example has a clause as modifier.
As far as the 'adverbial conjunction' goes - just because two constituents have a similar semantic effect doesn't mean they fall into the same word-class.
It's quite well established that as is a preposition in this case and it heads a prepositional phrase that is complement to the verb imagine (CaGEL p279).