Does anyone know how or if my @icloud.com address can be used as the default address for everything? I noticed that I can't use it as the default for Facetime, as I'd like to, or any other app. If I try to add the @icloud.com as my alternate email (and eventually as default) to my AppleID account, it says that it is already in use by another user. but that can't be possible because I own it, but can't use it?
It's already in use because it's the same account as the @me.com address. What you need to do is to arrange that you send using it as the 'From' address, thus ensuring that people reply to it. Please see this page: