A General informs his commanders his intent to invade well in advance before an operation. The plan was to remove the spirit, or a portion of it, to cause an effect. The event changed the rules of the game.
You make some interesting points about a pre-flood anarchy verses post-flood civil law being instituted.
If it was God’s intent just to wipe out wicked mankind he should have just used one angel to do the job. He went well out of his way to bring about the flood event. I believe his main goal was to change the atmosphere surrounding earth. The rainbow, fermentation and a gradual life span reduction were a result of a new atmosphere.