Generally, I have you on ignore but not this thread title, so I wanted to address a concern here but probably won't stick around much longer than to give a brief post.
First off, you are talking about a very specific doctrine:
What is Irresistible Efficacious Grace?
Your question moves beyond the walls of Irresistible Efficacious Grace, into the broader realm of God's Sovereignty, because specifically, Irresistible Efficacious Grace is about salvation for lost people and not the category that addresses their fallen nature ("Total Depravity/Inability"), nor that of how angels fell either, so a direct question into this doctrine would have to wade beyond the expression of what Irresistible Grace means and applies to directly.
Your question isn't really about Irresistible Grace, but about the broader idea of God's Sovereignty in not intervening and stopping the falls of men and angels. In effect, you are asking, it seems, "Why could man or angels have fallen, if God is irresistibly sovereign and could have expressed Grace in intervention, rather than remedial?" or made it so they wouldn't have wanted to do so, etc.
You could then ask Calvinists specific question, but it would more accurately reflect, I think, what you are trying to ask as well as invite all theologians to weigh in on God's Sovereignty in allowing the Fall in a more meaningful way, imho. The jump between the Fall with ensuing Depravity, to Irresistible Grace, iow, is a large one, from "T" to "I" in the TULIP acrostic.
In a nutshell, the answer doesn't delve into God's Grace being Irresistible at that point, but rather that God in Sovereignty, wisdom, Grace, and Love, chose how He would respond in the work of Christ, thus 'Christ' is the more specific answer to your question of 'why the Fall?" and "according to His own wisdom and Holy nature" concerning Lucifer's fall, because we are left with saying God only gives so much information to us regarding it. Grace, specifically, is the way God is regarding sin and How He responds to it. Grace, therefore, doesn't stop sin from happening, but rather is the eternal quality of God that responds to it to save. Rather, "Sovereignty" is what you are asking about when you are asking how a perfect being could Fall, and it isn't just Calvinistic in concern. We all address this about the same: God did as He, being Holy and Righteous and God, chose to interact with angels and men both in their perfection, and during their Falls. God is Irresistible in His Sovereignty as well, so you are asking the 'why' questions here, rather concerned with God's will and Sovereignty, I think. -Lon
