Just a shot it the dark here. Something caused the pulley bolt to snap. Could it have been one of the accessories the belt drive seized and caused the bolt to break? If that happened the added stress on the mounting bolt could have caused the damage to the threads in the timing cover. I would check all the belt driven accessories before putting it back together.
Reading your description of the work you did, I can't believe it was something you did to cause the damage.
Reading your description of the work you did, I can't believe it was something you did to cause the damage.