Fuel stabilizer is good to a point. The shelf life on most of the fuels out there today is around 30days, so a stabilizer is good to keep the fuel fresh a bit longer. Thinking it's going to hold your fuel for 6 months might be a stretch. Running the engine without a muffler may sound cool from a mile away but it can really damage your valves. You also mess with the back pressure when you take off the muffler. There is nothing but trouble doing that. At the track with open pipes the first thing done after a race is plug the pipe so cold air does not hit the white hot valves. Please put your muffler back on.
The flames coming out of the exhaust port is normal. Ever see an old WWII movie when the planes took off at night? Flames were coming out of the exhaust pipe. We see it all the time up north with snowblowers at night.
I would suggest putting things back together, throw some sea foam in the fuel and let it do it's magic. If you have issues after that we'll deal with them at that time.