The rule: if the pronoun is the subject of its clause and it is referring to a noun that is also the subject of the sentence, then it is safe to presume that there is no pronoun ambiguity.
In this sentence, while is a subordinator and the subject is the depressed values, the pronoun they is also the subject in its clause and it is referring to a noun that is also the subject so it's a clear reference.
For this specific rule refer to manhattan sc,the chapter about pronouns under the subtitle pronoun strategy.
And for 2: it's definitely the wrong answer (it has no reference and it is singular and in this context it cant function as an expletive.
hope this helps.