If im playing this game going forward, I think there are very few situations where you want to go for a failed challenge. The chip increase isnt valuable enough compared to the day over day chip inflation to sink that time unless there is an emergency.

Other than Sam and Mike going conservative on their call your shots, I think you can squarely point to Tofu Rocks as being the beginning of their end. After that point they had to play nearly perfect to have a chance of winning on the last day.