If you could give a few details of your hardware perhaps other customers could offer you some help/support. I respect that you are frustrated by your experience but you have not offered us anything much to enable help :(
In respect to your "10 devices registered" you do need to delete "ghost" accounts as you delete them. This is very possibly an "undocumented feature" of the Amazon ecosystem but it's quite easy to cure (I thought).
I am guessing you want to download a Amazon Music app for either an android smart phone or an Apple iPhone. If that is correct then I'm sure there should be one in the app store. I'll go have a look.
On message is sending me to amazon underground that's not even available. I can't download the app - If I send to 'the cloud'. I can't get to it from the phone. I can't send it to the phone because i cant bloody register the phone.
I sent a help message. It was sent from the phone I'm trying to install. Not the same email as the amazon account - reject. another waste of half an hour. Now what - forward the message to the other mail account and then to amazon - NO, amazon email address from anonymous email account I can't reply to.
Amazon says once per year, I can de register all then re-install. I have deregistered all but one. It won't de register and it's a laptop that has not been turned on over 2 years.
Fork in it. Screw this appstore and unsupported apps.