1. Not a huge advantage, but if you learn some of the sounds of MW3 it definitely helps. MW2 was the best game for a headset hands down. I could hear people coming from across the map.
2. Yes I get this all the time. The better player is always called a camper by the person that died.
3. Chatting does drown out the sound a lot. I will either turn voice chat low if I'm playing with friends or turn it off if I'm by myself.
4. Your own footsteps shouldn't be a huge concern unless you only play S&

. They are rather silent already. SitRep Pro works decently. Its super easy to hear people when they jump off things, walk on metal, pull pin on grenade, etc.
Overall the audio in this game completely sucks compared to MW2. I can still say my headset is a huge advantage though. You just have to train yourself to remember certain small sounds. Hope this helped you.