I've only done a few cooks on my wsm22, so don't take any of my info as gospel. I fill up the water bowl, if only because I figure that's what it's there for and I haven't noticed any reason to put anything else in there.
On temperature, I haven't had any problem keeping it under 225, or as high as about 285. Couldn't get much higher than 285 though. Whatever it settled to based on how I did the vents, it really liked to just stay there.
I've not measured the temperature difference between the two racks yet, but I would guess the lower one would be a bit hotter to some degree. I'll have to check that out next time I use it.