1:
I don't mind what the results end up being, but if 8 hours ends up passing the poll and being implemented, it's going to be the first consistent complaint (outside of any major bugs that may occur) and one of the first things patched. Might even be the reason people move on to a different game. If you start with smaller numbers and go from there, there will more than likely be less angry gamers and a solution won't be as heavily prioritized if the numbers were flipped.
2:
If there's a morning, why not an evening, dusk, dawn, midnight, noon, etc.?
Sometimes simpler is better, but you can fit many times of day in an 8 hour window.
3:
I suggest starting with only a 3 hour day/night cycle, 2 hours of day and 1 hour of night.
This allows future updates such as "new pokemon are available in the new morning cycle" or something similar if applicable.
Lengths can also be adjusted and branded as "buffs" if need be in future updates. Everyone loves buffs.