

Gotounge did say who actually poisoned the well
It was the heir of the neighbouring dojo who was koyuki age and had a crush on her but his aggressive and rough personality and couldn’t properly tell her his feelings and later on after finding out koyuki and hakuji were married he knew he couldn’t fight hakuji alone so he went out of his way to poison the well and koyuki died fast and keizo died hours later after her.
Fun fact - the guy was one of 67 men who was killed!!
(It was gonna be added in a panel but gotounge was too tired to do so)