The closest things we have to an "authoritative" commentary on the Westminster Confession are the two catechisms that were published alongside it.1 Both the Confession and those catechisms have attached to them "scripture proofs" that were appended somewhat hastily after the original writing of the documents, and which are sometimes criticized as being incomplete and/or too simple.
But these are the best we have available with which to answer your question. On the clause "They being the root of all mankind, the guilt of this sin was imputed," the Confession lists the following passages as proofs: Genesis 1:27–28; Genesis 2:16–17; Acts 17:26; Romans 5:12, 15–19; and 1 Corinthians 15:21–22, 45, 49. Of these, only the Romans and Corinthians passages are relevant to the second half of the phrase. In the Confession, they read:
12 Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned. 15 But not as the offence, so also is the free gift. For if through the offence of one many be dead, much more the grace of God, and the gift by grace, which is by one man, Jesus Christ, hath abounded unto many. 16 And not as it was by one that sinned, so is the gift: for the judgment was by one to condemnation, but the free gift is of many offences unto justification. 17 For if by one man's offence death reigned by one; much more they which receive abundance of grace and of the gift of righteousness shall reign in life by one, Jesus Christ.) 18 Therefore as by the offence of one judgment came upon all men to condemnation; even so by the righteousness of one the free gift came upon all men unto justification of life. 19 For as by one man's disobedience many were made sinners, so by the obedience of one shall many be made righteous.
1 Corinthians 15
21 For since by man came death, by man came also the resurrection of the dead. 22 For as in Adam all die, even so in Christ shall all be made alive. 45 And so it is written, The first man Adam was made a living soul; the last Adam was made a quickening spirit. 49 And as we have borne the image of the earthy, we shall also bear the image of the heavenly.
The verses in italics above (Romans 5:12, 19) are cited in answer 25 of the Larger Catechism, the one that most closely deals with this subject.
Some denominations do have commentaries on the Confession that they regard as authoritative; the RPCNA is one such example. But the broader body of Reformed theology does not hold that any specific commentator's viewpoint is authoritative, though some are clearly more respected than others. Commonly cited commentaries include Robert Shaw's (1845) and A. A. Hodge's (1869).