I was surprised to see the theories you came up with as to where 'break a leg' originated. I had it explained to me by a junior high student in a clown suit, but I guess if ya don't have the sources, it's no sin.
Back in the day, curtains were weighted down by heavy pieces of wood (these days we generally use chains) sewn into the bottom of the fabric. This kept the curtains from flapping around just any old way. I should also mention that curtains with wood in the bottom would be raised and lowered rather than pulleyed out from the wings.
After a performance, the audience would applaud, actors would take their bows, and the curtain would be lowered in front of the actors so the theater-goers would no longer feel obligated to clap. If, however, the applause continued, the curtain would rise again for a brief period and then fall again.
A very successful performance would prompt much rejoicing from the crowd, and the curtain would go up and down so many times that the pieces of wood in the bottom would snap or crack (conceivably from careless stagehands getting lax with the ropes). Hence, "break a leg" would be a wish for much enthusiastic applause.