Hi, what about giving the sound a tag like "enemysound" and before playing the sound, add a condition to check if that sound is already playing? :)
For the 2nd question, for each should be working, I'm on my phone now so I can't view you capx, but the problem may lie in some other conditions.
The last question, it depends on your final export format. For most exports, I think C2 automatically removes the irrevelavnt and unneeded formats on export.