What this discussion is missing is that nothing in creation is out of God’s control, and that includes Satan. Satan, meaning “adversary,” is the adversary of mankind, not of God. Whatever Satan is doing is still part of God’s plan.
No.
Sin is missing the mark in God’s Plan.
A creature with volition to perform His Plan or to rebel from it has the ability to sin. Sin is not acting per God’s Plan. It is anything other than God’s Plan.
Unlike humans, of which only two were ever made perfect, we all are born dead to Him spiritually. Once we become cognizant of Him and have the opportunity to accept Him through faith in Christ, we have an opportunity for redemption.
Fallen angels do not have a redemption solution for sin because they were created with perfect volition and eternal life.
God’s magnificent Plan required our condemnation before our salvation with eternal life.