How persuaded are you guys by that theory that Oberyn killed Tywin (well, was on his way to)?
See this link:
http://boiledleather.com/post/24196234491/tywin-lannister-dead-man-shitting
Oberyn knew everything there was about poisoning and they had dinner together. It also makes sense he would aim to kill the man who "controlled" the Mountain and let him off his leash. And there was a huge emphasis on Tywin's
super rotting corpse, to the point where it wasn't natural for corpses to rot that fast or badly. Tywin could have been poisoned, attempted to relieve himself (he was found on the shitter), which would have been impossible given the nature of the poison and got shot, which did relieve him and create the awful stench.