For Question 1, AlienBAIState.cpp has a function selectFireMethod() at line 1685; that's what I am using to answer you.
If the distance between the alien and the soldier is less than 4, the alien will try to Autoshot first, then Snap, then Aim.
If the distance is greater than 12, it will try to Aim. If it can't Aim shot, and the distance is less than 20, it will Snap.
Otherwise, the alien will try to Snap first, then Aim, then Auto.
Other than distance, the way shots fail is if they lack TUs. Nothing else is considered.
So to answer your question : if you set the TUs low enough, the AI will likely always select Autoshot when distance < 4, or if the distance is between 4 and 12 inclusive and it can't Snap or Aim shot.