Concern #2: I like to closely control the amt of salt I add when cooking. IMO, rough ridin' that up is one of the easiest ways to ruin a dish. I would imagine that would be more difficult with that brick.
#3, as you cook/clean the thing, it loses mass. As it loses mass, it will eventually heat differently and thus cook differently.
#2: salt a little less than normal. Learn from trial and error?
#3: true, but very, very little. You're more likely to lose it due to fractures in the block, which will inevitably happen, it's an inherent risk when cooking with them. That leads me to...
How many times can you cook on it before it's gone?
Most places I've read say a couple dozen or more. Done properly it should last a while, but eventually it IS going to break. All you can do is take the proper precautions and enjoy it while it lasts. Then, if you enjoy it enough, you can buy another.