Hi,
I was having the same issues and Apple Support couldn't really help me. So I just kept restarting my phone every time these issues came up, when all of a sudden after a restart the music app told me I have no music on my iPhone. So I checked the Usage Screen in the Settings App, where Music wasn't even listed anymore but the space (about 60GB) was still seen as used. I plugged it into iTunes and there it said I had about 50MB of Music and 60GB of "Others". 😮
Then I needed a drink, because I feared what I had to do: delete and restore my phone via iCloud Backup.. 😮
I tried restarting it a few more times, but that didn't change anything. So I put it in Recovery Mode and then restored it via iTunes. Then I selected "Restore via iCloud Backup". That was on Tuesday evening.
Now it is Thursday evening here and since it has about 31GB of 100GB restored and it is really super slow. I already spoke to the Apple Support again, but they couldn't help me and told me I needed to wait until it is finished, which can take a very long time. In the meantime I can't use my phone because Apps keep crashing, including the iTunes Store App, which is trying to download about 3000 songs right now… - I also can't use my Apple Watch, because Support told me that the Bluetooth Connection could also slow down the process. I deactivated iCloud Music Library because this might speed up the process of restoring Support told me, although there are still about 3000 songs about to be downloaded in the iTunes Store App… It has only restored 10 of 160 Apps on my device and I am really frustrated. Apple Support was really trying to help me, but there is obviously no other solution to this than waiting - even if it takes a week… (Maybe someone at Apple should think about how to speed this up, since memory spaces are becoming bigger and bigger and no one really wants to use cables anymore, especially not for daily backups… And when downloading from the iTunes Store on my Mac I certainly don't need 2 days for 30GB, so why does it take that long when restoring an iPhone?!)
I hope the restore is done soon and Apple finds a fix for this. I'm kinda anxious because I got my iPhone on launch-day and my warranty is going to expire soon (some days after iOS9 is supposedly released, which I hoped will fix these issues). I am an iPhone user since the first day the first generation iPhone could be bought and I am really disappointed right now. Sorry, I just needed to get this out of my system - hope no one else needs to go trough this annoying process due to these issues. Next time I will make regular backups via iTunes as well, just in case…😕