A - Part 2 :: I'm not sure if I understand your use case entirely, I have no idea what 'mqtt' is. But I can say that file uploading is planned for the future. Then you'll be able to upload files from inside the app directly to your storage. Does that answer your question?
B :: That's not possible no. Maybe someday?
C :: Only using the "anonymous" mode at the moment. Blockstack is still working on iOS support. So at the moment you can login using the anonymous mode, and then add a remote ipfs hash or AWS S3 bucket/directory.
D :: Like I described in my answer for
A, the songs are not copied/stored, the app only extracts the metadata and stores that. Your files stay where they are.
Thanks for all the questions! Let me know if I missed something or if something wasn't clear.
PS. There's also the about page, which you can find here http://ongakuryoho.com/about/ But I now realize that I probably should explain it a bit better.