Boondocking - There is a variety of boondocking options west of Zion, most are on BLM land, the sites along Kolob Road are on private property, but the owner allows people to stay here.
Restrooms -
Library - The library in Springdale is decent to work out of, there are a few spots where you can plug in and have a desk or a chair, the wifi is usually very slow, and there is no cell signal inside the library. Another option is the library in Hurricane (30 minutes west of Zion), there are plenty of places to plugin, but again the wifi is extremely slow.
Coffee Shops - Currently the only coffee shop in Springdale that has wifi is Deep Creek Coffee, it is always so busy that I have not been able to get a seat to even test out the wifi. A better option is to go to River Rock Roasting in La Verkin if you need fast wifi.
When to stay - April and May, October through mid November
Grocery Stores - Sol Foods in Springdale is a nicely stocked store, although it is exceptionally expensive, Whole Foods doesn't hold a candle to their outrageous prices. In La Verkin you will find a Lin's grocery store (Kroger) and a grocery store called Farmer's Market, don't let the name confuse you, it's a typical Western Family store, but you can find some good items here. There is also a Natural Grocers in St George.
Shower - The only shower you will find is at Zion Outdoor, it is $5 for a 6 minute shower.
Cell Signal - Varied, most of the BLM sites on the south side of Hwy 9 have a weak signal, but very good with a booster. The sites along Kolob Rd have a weak AT&T signal that can be boosted, but is very slow.
Laundry - There is a decent laundromat in Springdale or Hurricane.
Guide Books -
Favorite Hikes In & Around Zion National Park, Photographing the Southwest Vol.1: Southern Utah