I see many good ideas here, but some, I think, would not work well in a game setting.
War vs Battle: this wouldn’t work well for an unlimited siege style war, as a battle could start day 1 of the game, and continue until server wipe. Which with the purgatory mechanic, would result in people never being able to play.
Possible Solution: limit the purgatory rules to specific battles, for a specified period of time. Possibly based on the strength of the units involved?
War should be quick and decisive: this works well in most games, but in a game as large and complex as CoE is intended to be, it seems unlikely.
It is also historically accurate that Wars tend to drag on endlessly, sputter out, then burst back into flame unexpectedly.
If you start a war, you should see the consequences centuries later.
Medieval battles had few deaths: few deaths during the battle, yes. After the battle, groups of thieves would roam the battle field stealing from the injured and the dead. Frequently finishing off the injured.
And that was only the first part. More died of their injuries over the following days and weeks. Not to mention the civilians killed as sickness grew from the corpses left out to rot.
Modern medicine makes a huge difference when it comes to recovery.