And this is news how? Nintendo already said it was 2.5 hours for breath of the wild in January, I guess that means there was an improvement in optimization. And the average battery life for games would be in 3-to-6 hour range depending on how "demanding" the game was on the system.
As for the question on the system dying on the cradle the answer is no. There is circuitry that switches the system to use only A/C power when plugged in. Laptop, cars, etc all have this where if there is an A/C current available the battery will only charge and most of the system will run on A/C because Laptops would die when plugged in if this wasn't true. Playing 3D games on laptop drain the battery in less than 2 hours and most batteries take longer than that to charge.
