enter their username and password
no they wont Apple will let you use an iTunes account to purchase items from their iTunes store.
Yes, you can. You could buy songs off of iTunes on that person's computer, but they wouldn't sync into yours. If you bought something off iTunes, synced it with your iPod, then hooked up your iPod onto your computer, then it will overwrite all the songs on your iTunes to the ones from the other person's iTunes or vice versa. You can use your apple ID on someone else's iTunes, but I don't suggest it.
iTunes only allows you to download purchased music once! You have to move the files from the computer you purchased them to your other computer manually, by either burning them to CD or on a pen drive. You'll still need to use iTunes to listen to the files, unless you were lucky enough to purchase DRM Free music.
Yes you can
Yes if you get his or her itunes password and username.
t account
That is fraud!
If you know there password
You can have as many as you want. But if someone already has an account, you can use your ipod/iphone on that account and won't take the songs on that persons account. It will even recognize that you are new to itunes.
no you need to set up an account before you can use a gift card
You should be able to use your iPod and iPod Touch on the same iTunes account on both computers.
your apple account