its a known issue that when you "update" downgrade via IPSW, you lose message history and there is no way to get it back unless you restore a backup.
also, as for the music app issue, its also a known bug. i had this issue yesterday when i reverted back to 9.3.2. if you look at the music app in app switcher, it shows the iOS 10 music app. also, it force closes when you try to navigate the app. your only option is to make an iTunes backup, restore to 9.3.2 as a new device, then restore your backup. your music will work again after you resync it.