"But the Father's will needn't necessarily have been the Passion."
How do we know this? I think we don't really. I've heard it said that God could have willed to save us in any number of ways, but I wonder if that is really true. In other words, it seems at least possible that His nature plus the nature of our perilous situation *required* the Incarnation, Passion and death of His Son- that if He was going to save us at all, it HAD to be done in that way in His view. Do we really know enough of the inner life of God, of how His mind works, to so confidently say that He did not have to do it in this way? I know theologians say that any theandric action offered for our salvation would have been enough. To me that has always seemed to be talking very considerably beyond our knowledge of God. If Divine Revelation is silent on the subject- and I think it is- then we simply do not know and cannot reason our way to such a confident declaration. Or so it seems to me. Of course, I submit to whatever the Church pronounces on the topic, but it hasn't been defined, has it?
I don't know offhand exactly what has been pronounced or defined, with what authority, on this question.
I would say, though, that once you add something like "plus the nature of our perilous situation," you're already beyond that first acceptation (is that a great word or what?) of "necessary" -- viz, "anything which of its nature cannot be otherwise" -- applied to God.
And I think, though what do I know, the "any theandric action offered for our salvation would have been enough" argument can be understood as a direct corollary of Divine omnipotence and transcendence. Creation quite simply cannot impose the necessity of compulsion on the God Who created it.
At the same time, though, it seems to me that any other theandric action would have had different effects; if we had been saved by Jesus picking up a pin, for example, then we most likely wouldn't have crucifixes in our churches. So the Father's willing the Son's Passion was necessary from presupposing the end of our being saved in the particulars of history.
I'd say God has revealed enough about Himself and our relation to Him that we can know He could have saved us through other means. It strike me as plausible that the salvation obtained through other means would not be identical to the salvation obtained through Christ's passion and death, in which case the specific Gospel actually preached can only have been preached by a Son Who had to die.
At the same time, the notion that God's very nature necessitates our salvation according to the specific Gospel actually preached seems contrary to the de fide declaration of God's absolute freedom.