I've given he a name to make for easier reference:
Bob sighed and replied quickly, as if he had had only a few seconds'
time before John changed his mind.
This sentence is not wrong but the following might be better:
- Bob sighed and replied quickly, as if he had only a few seconds
before John would change his mind.
The past perfect in the original example makes it sound as if Bob might have already had the seconds. But in this actual situation these seconds are about to occur, they haven't occurred already. I think would representing a future in the past is better than changed here. It clearly orientates the notional sequence of events. Lastly, we do not really need the word time in this sentence. It is a bit confusing because we expect to see time used this way in preposition phrases using the preposition in, where it represents a point in time, not a period:
- I'll phone you in a few minutes time.
This sentence isn't a conditional. The Adjunct as if he had ... mind is an Adjunct of comparison, not a conditional Adjunct. Notice that the truth of the main clause Bob sighed and replied quickly does not depend in any sense on the as if clause being true. We can compare it with:
- Bob sighed and replied quickly if his interrogators asked him embarrassing questions.
In the sentence above we get the impression that Bob's sighing and answering is caused by the interrogators. The questions are a precondition for the sighing. This is very different from the Original Poster's sentence.
The Cambridge Grammar of the English Language (Huddleston & Pullum, 2002) classify as if as a compound preposition. CaGEL notes that when the main clause is present tense, the verb forms in the as if clause may be backshifted (p 1152). In other words we may see the forms we see in if-clauses in so-called remote conditionals:
- She always looks at me as if she hated me. (as if- clause as comparative Adjunct)
- She wouldn't be so nice to me if she hated me. (if-clause as conditional Adjunct)
Notice here that the main clause must have would in the remote conditional.
Now, although we often see a modally remote past simple in as if-clauses when the matrix (main clause) verb is present simple, we only very rarely see modally remote past perfects in as if-clauses, when the matrix verb is past simple (CaGEL, p. 1153). Very often it is not possible:
- She looked relaxed, as if she skated every day.
- She looked relaxed, as if she'd skated every day.
In the second sentence above we can only understand as if she'd skated every day to refer to an earlier time, before the time we are actually talking about. It cannot be interpreted similarly to the first example. It cannot refer to general time at the time that she looked relaxed.