There's the first item of substance I've noticed you get right. :-)
Yes, when people debate "the Jews killed Him" "no the Romans killed Him" some Christian will point out "no, our sins killed Him" and it will eventually hit home to all "I killed Him".
While all of those things are true - and the later are necessary realizations to come to - there can be no doubt that God did it. And knew that He would do it from the beginning of time.
All of which, of course, is irrelevant to this particular discussion since the debate is over what "it" is and how it could be "finished" at calvary if the sacrifice was accepted today. Your post would serve to defeat mom's point.
Ahhh and we have perfect agreement .The cross was first and foremost a perfect vindication of the Holiness of God.That however does not negate that the cross was the method of reconciliation and salvation ordained by God .
Both things can be and are true .
We are resurrection people..
Rom 1:4 And declared [to be] the Son of God with power, according to the spirit of holiness, by the resurrection from the dead: