Pletch,
You've done an incredible job of researching this issue, and debunking the thought that placing the priming powder away from the vent should be
read by everyone shooting flinters. I do just about everything you do in your method, except that I don't carry my air compressor to the range. I wipe between shots, and clean the vent with both a brass rod and pipe cleaner AFTER I've wiped the barrel, in case that forces fouling into the vent. My vent is positioned even with the top of the pan, although it is about 1/16 back of center. From now on, I'll be sure to bank the priming powder (Goex 4F) close to the vent, leaving the pan fully covered with powder to ensure good ignition. Your use of the heated wire took the variable of flint sparks out of the equation, and that's something the shooter has to be careful of--the condition of the flint and how evenly it strikes the frizzen.
Do you think that the main powder charge should be pushed out of the vent with a pick? I use 2F in the barrel, so would it prevent the fire from effectively reaching the main charge if some grains of it are left in the vent after loading?
Your use of a camera with a short barrel is very instructive, and saves me the trouble of looking down the barrel which, mine being 42 inches long, is very hard to do. Once. I need to have at least one eye.
All shooters should thank you for your very clever research. I wish I could get to the range tomorrow.