adverbial clause acting as adjective

Consider these sentences, please:

1) Imagine Robert Redford when he was a child - that's what John looks like.

2) Imagine Robert Redford as a child - that's what John looks like.

Question 1: Can I say that the bold clauses in 1 and 2 are modifying the noun "Robert Redford" adjectivally?

Question 2: Comparing 1 with 2, can we say that "as" in sentence 2 is not a preposition, but rather an adverbial conjunction of time? For example:

Imagine Robert Redford as he was a child - that's what John looks like.

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:

guide price

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).