You are trying to rationalize something that was a decision of the Father. The fact that it happened is enough for us. Whether it was necessary or not depends on to "what" you think the Father was bound by. If you say He "bound" Himself, then you say He didn't have a free will decision - that is not surprising - considering that fate is so important in your scheme of things.
What external force? I'm saying it was God's INTERNAL requirement of justice (His nature) that set the standard here.
Does His nature REQUIRE such an extravagant show of love? Or was it a free will decision? Now, it appear that you are saying God does not have a free will, either! His "nature" made Him do it?
Regards
Of course God had a free will decision. He decided to let the serpent into the Garden, all the while knowing what would happen, and also knowing all the ensuing consequences if He wanted some of His creation ultimately in Heaven with Him. He knew from the beginning that if He allows man to fall that only one thing would be able to save him (man). God decided it all.
Does His nature REQUIRE such an extravagant show of love? Or was it a free will decision? Now, it appear that you are saying God does not have a free will, either! His "nature" made Him do it?
God's justice requires what it requires, whether an act of love or wrath. If God had decided to let all men be lost, then that would have been fine with God's justice. But He didn't. He wanted to save some. OK, how can God do that while remaining true to Himself? Answer - He takes the punishment in the place of those He wishes to save. What is the punishment for all sins? Death. That's God's own rule, so He goes by it. In order to be true to Himself, Jesus had to die for some to be saved.