You seem to place the forensic element as something both prior and subsequent to union with Christ.
Our sins were imputed to Christ. He wrought righteousness for us. That is forensic. He wasn't really made a sinner. It was a legal relation He bore in our place. We are united to Christ by faith on the grounds of His work.
Additionally, stating that real benefits are dependent on a prior forensic judgment, and calling sanctification a fruit of justification seems to be different than the Westminster Standards presentation of Justification, communion in grace (WLC 69), and even its definitions of Justification and Sanctification. Where in Scripture and the Standards is this idea?
Romans 5:12-21. V. 19 clearly states, "For as by one man's disobedience many were made sinners, so by the obedience of one shall many be made righteous." 2 Cor. 5:21, "For he hath made him to be sin for us, who knew no sin; that we might be made the righteousness of God in him."
WCF 10.4, "This office the Lord Jesus did most willingly undertake, which, that he might discharge, he was made under the law, and did perfectly fulfill it."
WCF 10.5, "The Lord Jesus, by his perfect obedience and sacrifice of himself, which he through the eternal Spirit once offered up unto God, hath fully satisfied the justice of his Father, and purchased not only reconciliation, but an everlasting inheritance in the kingdom of heaven, for all those whom the Father hath given unto him."
WCF 11.1, "Those whom God effectually calleth he also freely justifieth; not by infusing righteousness into them, but by pardoning their sins, and by accounting and accepting their persons as righteous: not for anything wrought in them, or done by them, but for Christ’s sake alone; nor by imputing faith itself, the act of believing, or any other evangelical obedience to them, as their righteousness; but by imputing the obedience and satisfaction of Christ unto them, they receiving and resting on him and his righteousness by faith; which faith they have not of themselves, it is the gift of God."