The idea that "Jesus was plan B" doesn't make sense from a theological or a logical standpoint. If God is Omnisicient, then He knew what would happen before He created everything. He knew Adam and Eve would choose to disobey when tempted.
To claim that Jesus was "Plan B" would indicate that God is not Omniscient, or that God failed. Both go against Biblical doctrine.
I believe it's possible that God could have prevented them from sinning if He'd wanted to, but that would have meant we had no free will. If God had wanted to ensure we all remain sinless, He'd have done so.
Some Scriptural/doctrinal arguments:
- God desires obedience.
1 Samuel 15:22 (KJV) And Samuel said, Hath the LORD as great delight
in burnt offerings and sacrifices, as in obeying the voice of the
LORD? Behold, to obey is better than sacrifice, and to hearken than
the fat of rams.
However, without free will, there is no obedience, so there is no such thing as good, because evil is an impossibility. I believe that free will is necessary because without it, there simply is no good.
The highest form of "good" is perfect, willing obedience to God, which is something only accomplished by one person ever.
I believe that what we have is the best God could provide us without eliminating free will.
If He is omniscient, then He knows not only what Adam and Eve would have chosen to do, but each and every decision every single living being that has ever and will ever live would make.
Bottom line: Unless God is fallible, Jesus was plan A all along.
Scripture also backs this up. Before the foundation of the world, we were chosen to be adopted by Jesus Christ Himself.
Ephesians 1:4-5 (KJV) According as he hath chosen us in him before the
foundation of the world, that we should be holy and without blame
before him in love: 5 Having predestinated us unto the adoption of children by Jesus Christ to himself, according to the good pleasure of his will
1 Peter 1:20 (KJV) Who verily was foreordained before the foundation of
the world, but was manifest in these last times for you,