i must've got a very bad spool then or i'm alot stronger than i thank. i've broke it numerous times on some deer hair flies.
Actually, I bet your bobbin has a very slight burr that is cutting the thread when you really crank down hard on it. Steve can get you my favorite bobbins, the Griffin Ceramic that will fix this problem.
On constructing flies for a one fly tourney, I would suggest superglue over sally hansen's, because it takes less of it to really hold well, and if you're really gonna do it right, you will be applying the glue to the fly over 10 times. Basically, every time you tie something in, like a tail, rib, shellback, you name it, put a stripe of superglue on the thread wraps. When u go to finish the fly, do 2 or 3 whip finishes and coat all of the tie off with superglue, making sure to keep the hook eye clean.
On thread size, especially for the parachute, I would suggest using a slightly smaller thread, more wraps than you would normally use for a given step, and then let the super glue add the strength. Super glue works really well at holding thread wraps together. Also, picking a thread with less wax on it will help, as the glue will be able to penetrate it better.