The translation you propose as the “better” one, is actually the same I used in my quotation:
Now, consider the following question:
When did God decide “Christ’s salvation plan”, i.e.: that the Lamb will be slain for the sins of humankind?
In the perspective of God “outside time” the answer seems to be:
From all eternity, as time did not yet exist.
Now, as you very well state, this cannot mean that humans were “predestined/created to sin” by God.
But then, what does it mean?
You propose:
“humans… would have had to start sinning first” and then “Christ’s salvation plan became operational for all people in history”.
I basically agree, but this should not be understood in the sense that “a human act (the Fall) is the cause of a divine action, God’s decision of salvation”.
In my view the correct way to formulate things is:
God’s decision to redeem us is a consequence of God’s pondering that humans can sin, and it is God’s response to this possibility from all eternity, before sin happens. Thus, if sin happens, the fact that “the Lamb is slain” is not caused by sin, but by God’s will to redeem us:
No one takes my life away from me. I give my own life freely. (John 10:18)
It is important be aware that it is God’s decision what founds reality. All possible decisions humans can make that are relevant for eternal life (the only reality that matters for everlasting happiness!) are present in God’s mind, and God has a plan for each possible decision tree, and all these plans co-exist in God’s mind.
Therefore, the very fact that the world and humankind exist, results from God’s decision to create humans ordered to eternal life. But this decision is concomitant with the decision to redeem them if they sin. Thus, it is also fitting to say that the Lamb was slain before the foundation of the world, very much in agreement with:
Who verily was foreordained before the foundation of the world, but was manifest in these last times for you. (1 Peter 1:20).