A water knot is the best knot for joining two ends of webbing, I wouldn't recommend any other knot except for maybe the beer knot, but that's certainly not going to save you any time. You don't need to tie back up knots either, webbing doesn't slip like rope does, in fact I've never known webbing to slip at all, and I've set up a lot of slacklines using 1" tubular webbing.
A figure 8 follow through is more likely to slip than a water knot, because the webbing can't lay flat, so there's less friction in the knot. If you're concerned about slippage in your water knots, leave longer tails.
The best thing to do to aid in the quick untying of a water knot, is to tie either a piece of cord, or a rappel ring, or a carabiner into the knot that you can use to leverage on when untying. That's what I do when I'm setting up anchors for a slackline, as a slackline will tighten a water knot a lot more than any top anchor ever will.
There are other options for using webbing as an anchor that don't involve tying and untying a knot:
You can simply throw your pre-tied 1" webbing sling around the tree and join it with a carabiner:
You could also put a girth hitch around whatever you're anchoring to, but you have to be careful with girth hitches, because if tied wrong they can actually put twice as much tension on the webbing when tied one way vs another (think of a basic pulley system).