Coal usage, probalby not bad but depends on how hard you are running the stove. Yes, the flames can touch the top inside of the stove.
As long as you are not pushing hot coals off the end when cranking, you have it set pretty good. About 1" of ash at max burning.
Yes, if you don't have the baro set correctly. You will loose more heat up the chimney in stead of in your stove/house...
Get a magnetic thermometer and put it on your pipe, then you will know exactly how "HOT" the pipe is. The baro should be adjusted with a draft gauge to maximum your efficiency of your stove and not have too much or too little draft.