On the response to verses such as Gen. 15:6, I'll let John Gill respond from his commentary. On Gen. 5:16 -

"And he believed in the Lord,.... The Targums of Onkelos and Jonathan are,

"in the Word of the Lord;''

in the essential Word of the Lord, in Christ the Lord his righteousness; he believed in the promise of God, that he should have a seed, and a very numerous one; he believed that the Messiah would spring from his seed; he believed in him as his Saviour and Redeemer; he believed in him for righteousness, and he believed in his righteousness as justifying him before God:

and he counted it to him for righteousness; not the act of his faith, but the object of it; and not the promise he believed, but what was promised, and his faith received, even Christ and his righteousness this was imputed to him without works, and while he was an uncircumcised person, for the proof of which the apostle produces this passage, Ro 4:3; wherefore this is not to be understood of any action of his being esteemed and accounted a righteous one, and he pronounced and acknowledged a righteous person on account of it; for Abram was not justified before God by his own works, but by the righteousness of faith, as all that believe are, that is, by the righteousness of Christ revealed to faith, and received by it: what is imputed is without a man, and the imputation of it depends upon the will of another; such the righteousness of Christ without works imputed by God the Father. This is the first time we read of believing, and as early do we hear of imputed righteousness."

John Gill on Rom. 5:1 and notice carefully the first words of the paragraph:

"Therefore being justified by faith,.... Not that faith is at the first of our justification; for that is a sentence which passed in the mind of God from all eternity, and which passed on Christ, and on all the elect considered in him, when he rose from the dead; see Ro 4:25; nor is it the chief, or has it the chief place in justification; it is not the efficient cause of it, it is God that justifies, and not faith; it is not the moving cause of it, that is the free grace of God; it is not the matter of it, that is the righteousness of Christ: we are not justified by faith, either as God's work in us, for, as such, it is a part of sanctification; nor as our work or act, as exercised by us, for then we should be justified by works, by something of our own, and have whereof to glory; but we are justified by faith objectively and relatively, as that relates to the object Christ, and his righteousness; or as it is a means of our knowledge, and perception of our justification by Christ's righteousness, and of our enjoying the comfort of it; and so we come to..."

On the difference between God's decree to create and then his actual creation, Elder David Pyles -

"This thinking is irrefutable, though some have presumed to overthrow it, saying, for
example, that since God’s plan to create does not imply creation, neither does His plan to
justify imply justification. While this complaint has a valid point, it is not a point that
would refute what the English Baptists intended. A distinction must be made between an
action of God and the state of His mind. God might plan an action or change of action,
such as the introduction of a Universe or the regeneration of a man, but it is logically
impossible to plan a change of mind. There is no sense in saying that God planned to
view the elect as unrighteous over some span of time, but simultaneously planned to
change His mind and view them differently thereafter. He indeed viewed His elect as
righteous in Christ from eternity, and with no other position will one make sense of the
fact that God was glorifying His deceased elect long before Christ died and before there
was a gospel to preach."
http://www.pb.org/PBDocs/JustificationAndBaptists.pdf