That's the main problem, everything else is minor. Check out this Park Tool site on torque specs. The crankarm bolt requires 25-33 lb-ft of torque! No way you're going to do that with a short 6" allen-key. even with an automotive-style 3/8"-drive torque-wrench with 12" handle, there's still a significant amount of force needed on the wrench. At 33 lb*ft, it seriously feels flexy like you're gonna snap the bolt!

I worked in a shop for 10-years and in the beginning, we had a rash of bikes coming back with loose crankarms. People tried all sorts of tricks, thread-lockers, serrated lock-washers, etc. All that was just fixing the symptoms, not the real problem: insufficient torque. We implemented a build-sheet for all the bikes with specific areas that needed to be torqued and signed off. Turns out the mechanics were only tightening the crankarm bolts to only about 1/2 the required tension. In a single month after implementing the build-sheets, our returns from loose bolts went down to only 5% of what it had been previously.

In the OP's case, that crankarm is most likely hosed for good. Depending upon how long you rode on it with a wobbling loose crankarm the 1st time, you may have gouged out the taper in the crankarm. Even if you did crank it on tight enough, the mating surfaces are so bad that the arm will still come loose. THe fact that it came loose a 2nd time is a sure sign that the crankarm is completely toast. Best to save your time and frustration and just get a new crankarm.