Why does God know what is required to reconcile fallen man to Himself?
Because He is holy, righteous, omnipotent, and full of all knowledge, God understands the exact way to restore fallen mankind. He never violates His own principles, so His provision flows from the finished work of the cross, where He did not spare His Son but gave Him to purchase mankind back to Himself.
Les explains that while God is sovereign and could act however He wanted, He never goes against His own principles. God is holy, righteous, omnipotent, and full of all knowledge, so He knows what has to be done to reconcile fallen man back to Himself. This reconciliation is based on the finished work of the cross, where God did not spare His Son so that He could purchase mankind back to Himself. In Romans chapter 8, verse 32, the point is that everything God now has done and is doing on our behalf rests on that finished work.
He that spared not his own Son, but delivered him up for us all, how shall he not with him also freely give us all things?