You should only need to worry about mites if you live in a mite infested area or if you get a WC, but if one or both of those two applies to you than yes, bark is a good breeding ground for mites. The bottom line is this: if there are no mites to begin with then there are no mites to begin breeding in your bark! I've never had mite problems myself.
You can "spot clean" individual "poops" every 4-7 days because bark is a very clean substrate that absorbs and isolates messes and "poops". If you are independently wealthy or have a never ending supply of bark than you can change the substrate every week. If not, than just change it every 6 months to a year. Make sure you change it if it begins to smell like ammonia though!
I don't use newspaper so I can't tell you about how thick the layers should be. I do know that newspaper must be changed every day and doesn't hold moisture.
Hope that helps!