Well, there is much misunderstanding here so I'll keep simple. The sinners prayer is a man made concept. No where in the Bible is the idea found.
As for doing God's will, what is his will? That we believe in Jesus, and repent of our sins.
What makes a person saved? That they believe in Jesus. Not some prayer of acceptance. Believe in your heart, and confess with your mouth. Confess what? That Jesus is Lord. Now that there is belief, repentance begins and sanctification, and this is God's will. Anyone who does this is doing his will and their prayers are heard.
So you choose to disregard the Bible because what man teaches that isn't in the Bible. Because the sinners prayer contradicts a passage in the Bible. So you choose the man made idea over the Bible and then say the Bible is the man made idea. This sounds a bit illogical.