1. I've left yeast in the fridge 2 days once before and didn't notice any difference from when I left in in there overnight or the same day. I don't think the difference in viability from 1 - 3 days makes much difference on a homebrew level. As mentioned by travjohn above, I don't always cool and decant my starters. It really just depends on the beer. If I'm making a lower gravity beer, I may decant. If it's a bigger, darker beer, I just throw it all in. I've done this each way with an American Wheat beer and I couldn't tell a difference.
2. Mr. Malty can help determine how much slurry from washed yeast to use. Select the "repitching from slurry" tab and I usually slide the "yeast concentration" slider closer towards the "Thick Yeas" side since I will decant most of the liquid. When I wash my yeast, I usually store it into 4 different mason jars which have ml readings making it easy to estimate the amount of yeast in each jar. Depending on how much I will be using over the next week or two, I will either take a jar and make a starter or just throw in 2 jars, which usually will have 40 - 60ml of yeast slurry. With Mr. Malty, play with the "Harvest Date" in the "Repitching from Slurry" section to see how time changes the viability of the slurry.
3. As far as I know, you are correct with pitch rates with ester/ fusel production. Less yeast increases the ester/fusel production along with more diacetyl, which depending on the beer is what you may want. Higher pitch rates is less ester/ fusel. Fermenting on the lower side of the temp scale I think does help reduce esters, but I don't think you risk off flavors from poor fermentation. You may risk stuck fermentation if it gets too low, but I don't think it throws off flavors other than maybe a less drier beer. Ferment too high and you are in danger of some bad fusel production for sure.
I don't consider myself a yeast expert, so please if I'm wrong with any of this feel free to correct me.