The amount of charcoal is directly related to what you're cooking. Doing 4 butts for 20 hours will take closer to 16lbs, while a single rack of ribs for 4 hours will only take 5lbs.
So 10lbs isn't out of line.
I tend to cook the same way no matter what I'm doing.
I use whatever charcoal was left from the last cook. I stir the charcoal to clear all the ash and dump ip. Then I top the ring up with fresh charcoal. Then I light 20 or so in the chimney and dump them on top. I cook for however long I need, at whatever temp I need, and add a few more charcoal if I need.
When I'm done cooking I close all the vents and let the fire go out and repeat for the next cook using whatever charcoal was left unburnt.
I've read posts about bad things when using partially burned charcoal but I've never experienced anything bad due to it. I think its the bad smell of igniting charcoal that has people worried, but I get that smell from fresh charcoal as well and it's never been imparted to the food.
As for achieving temp, I have some trouble getting up to temp as well, but after a recent trip back to Arizona, I decided it was due to altitude. In AZ I had trouble keeping temps down, here in Colorado I have trouble keeping temps up.
About 4 months ago I purchased a BBQ Guru (refer to the modifications board for many threads about the guru) and I've had little trouble with temperatures since.