Cedar is actually toxic to ALL small animals. Are you sure it isn't aspen or pine? Either way, I wouldn't use the reptile carpet. Re-usable substrate in general is just no fun. It gets nasty really fast and it will become a pain trying to get the smell out. My favorite substrates are aspen or paper towels.
Nothing wrong with misting the tank, as long as the substrate doesn't stay soaking wet for days and days, its not a problem. You can try spraying the snake directly but they usually don't like that too much.
What are you using to measure the humidity with? If it is one of those stick on round things, then you probably aren't getting an accurate reading of the humidity (or temps). You want something digital to read temps and humidity. My personal favorite is the accurite weather station which isn't even sold at a pet store. I got mine from the home depot but other people have found them at lowes or walmart. They are generally found in the outdoor section for around $12.
Chances are the pet store employee gave you a lot of bad information. Do yourself a favor and don't listen to a word they say. Even if they are trying to help, they usually have no idea what they are talking about.