No
Yes. Just click "buy" and drag the song over to your playlist.
Yes.
yes
No. If you are downloading your computer needs to be on
When iTunes is upgraded, it preserves your music library so just run the upgrade process and you'll be fine
If I am right, there is no way to change email address without losing personal information. You have to invite everyone again and all your emails cannot be read by your new account. It is the worst thing to do when you change your email address. I advise you to look for another way to solve your email problem.
If think that this doesn't work because I've tried it before
You can't. You have to pay or risk losing your account.
World of Warcraft: Yes, you can change to other realms and create characters to play without losing the characters on your other realms. Just remember, there is currently a 50 characters per account limit to each account, and only 10 (11 when Mist of Pandaria releases) characters per realm.
You don't have to upgrade iTunes, you just can set your ipod touch to a "replacement" even though it's not.
You cannot change it after it is chosen.
Log-in in to your iTunes account on your new Mac and then sync as normal. You'll be asked if you want to copy stuff from your iPhone to Mac (which you probably want to do) and then you should be able to continue as normal. Remember to authorize your new Mac with your iTunes account as well.