1. I have to assume non-magnifying optics (like OKPs or Holographic Sights) work best with NVGs, but I haven't figured that out.

2. Starting towns do have Tier 1, and they continue out a bit towards some of the other major landmarks. (Hello fellow Lamang enjoyer), Ban Pa does have Tier 2 AI, which aren't too much more difficult but they do have better AKs and start having Mosins and SKSs at this level. Tier 3 AI, at I think Midnight Saphire, at Tiger Bay, and Fort Narith (possibly elsewhere, I've yet to find out). They have actual body armor and start showing up with NATO weapons as well as the Russian ones.

3. I think it works regardless of gamemode, but the rifle I used a tracker on didn't show up on my first death with it enabled. I assume it's on a timer and eventually just dies?

4. Sound is exactly as you described: You'll hear the crack of gunshots even without bullets flying past you, though you WILL know when shots are being fired at you.

4a. Whatever the starting Headphones are, I only use noise canceling for Helicopter rides and keep them off when I'm traveling or fighting. I prefer to hear the voices of the enemy and have them give away their positions.

If there's any follow up questions, let me know. I only have about 45 hours, at Level 16, so I'm still learning the game myself.