1. No you shouldn't have to worry about catching fire, if that is your question.
2. You should be able to achieve 55% humidity without soaking reptibark, unless you are in a super dry climate. Wet porous materials will have a higher tendancy to harbor fungus and bacteria. Are you having problems with your humidity? If so just a light misting with a spray bottle is usually enough to increase it.
4. No not necessary for most people, but it all comes back to your individual set up and what your current measurements are.