Do you have a barrel adjuster on the end of your brake cable housing? If not I would recomend putting them on. You just turn it to lossen you brakes to take off your wheel then tighten back when after you put it back on. The only other way I know of is letting air out of your tire, But who wants to do that each time?
Don't confuse great stopping power with reduced breaking power modulation range.
I'd bet the brakes will stop just as well if they were more open, however it will require greater break lever travel, which when you are used to it has some advantages.
Ditto. When I adjust my brakes (usually after taking the wheels off the frame to clean the rims, etc.) I generally put them very close to the rim. They 'feel' great for the first couple days, but they're actually more effective a day or two later once they've loosened up just a tad.
Low profile cantis like the Avids are usually best set up with more post exposed toward the rim. Your friend gave you a good tip, try it. Adjustment is easier, but the downside is that sometimes it makes them more prone to squealing or stutter. I hate low-profile cantis...