> if [...] 20 years of peaceful compromise would have ended slavery
There's no way to even estimate such things beforehand, so the calculation you propose is pretty useless.
I'm not saying that the decision for the war was right or wrong, but that you can't answer the question with a seemingly objective multiplication of numbers (it's the garbage-in-garbage-out principle in action).
What is the appropriate life per slavery-year exchange rate? Because if for one less year of slavery you would spend 10,000 human lives, and 20 years of peaceful compromise would have ended slavery, then your war was a mistake.