4. Is the "poor man's copyright" (mailing it to yourself) a valid way to protect
your music? NO! You should register it with the U.S. Copyright Office.
Anything you create and put your name to online is covered by copyright. Anything at all. You don't need to declare it copyrighted (though it's a pretty good idea, and you don't have to pay for the privilege). Mailing it to yourself is a way to prove ownership and is a totally viable way of proving copyright - but unless you get ripped off by a high-profile company, you won't need it anyway.
Copyright claims are civil cases, it is up to the defendant to prove they created the material, not the accuser.
Copyright offices are just a more elaborate way of doing this - the office takes care of the material for you, adds a few extra services like managing flat-rate licenses, handling royalties and also provides holds and maintains your copyright even after you die.
Nobody on NG should even consider shelling out for commercial copyrights on their tracks, firstly because it's a waste of money, secondly because anything you post on NG is automatically covered under the FREE creative commons license, which is a legally recognized free copyright license, so your paid-for copyright wouldn't apply to the work anyway as released on NG.
TL:DR:
Anything you make is yours legally, regardless of whether you register a copyright or not. If somebody uses it outside of your terms of use, plagiarizes it and claims it to be their own or otherwise, you still have the legal rights to stop them.