This is not God’s “plan B”.
I don't believe God ever needs a plan B either - but we do have to go where logic and Scriptures take us.
God knew what choice Adam would make before God said “Let there be light.” and God the Father had already determined to send God the Son to redeem a people that God the Holy Spirit could indwell and unite with THEM ... all for the greater GLORY of God.
It is like when Joseph’s brothers sold him into slavery. It was their idea, but it was GOD’s plan from before they were born.
Before "Let there be light" doesn't conclude much more than being before human time - but there are several things happening in God's mind sequentially, all before human time, before the foundations of the world. What we're specifically focusing on is Rom 9:11 and God's sequential partitioning of moments in His mind as
before and consequently
after any man's good or evil.
God never said He decreed the brothers to sell Joseph into slavery
before any of their good or evil. God instead says He did factor in their evil to then turn it for good. Of course, all this is before human time but still
after their own evil in God's mind (unless someone erroneously wants to argue for God determining to cause their very evil). However, this is not what God says with respect to His election of grace - He particularly qualifies it as being before any man's good or evil. This necessarily means He hasn't factored in Adam's sin in His mind at the moment of electing His people. He could very well complete His election and the very next instant foresee and decree all things until the end of time factoring in man's evil, but it just can't be at the same moment. If it were so, we'd be going against Rom 9:11 and Scripture cannot be broken.
It would've been so much simpler if Scriptures had stopped with just the passages pointing to justification by faith alone - but James does link works with faith and then we need to interpret what that correlation exactly is, whether evidential or causative. Similarly, it would've been simpler if God said He'd determined everyone's fate with no offer of salvation given to the non-elect - He's done it before with the non-elect angels and revealed it, so this wouldn't be impossible to accept in man given the precedent. But God does reveal His desire for non-elect man to repent and live and He does reveal His supernatural works in them towards salvation for a time until they fall away.
If we have to reconcile all this together, at the moment of His election of grace, God must've known there would be an inevitable fall
but without factoring in Adam's or anyone else's acts of evil / unbelief. IOW, God must've known about the fall based on Himself and not any man's acts. And we could explore trails there (karl barth's creational entropy settles it for me personally) - but we still need to hold Rom 9:11 to mean what it says.