Did you check your song count? They might not be lost (certainly none of mine were and they vastly outnumbered my digital buys) but simply listed in some other tab? There's still a "song" tab, they just moved it into the groupings list alongside artists, albums, songs, Music videos, genres, Composers and compilations.
Turn off the Apple Music option in settings in iOS 8.4, under the Music setting.
I agree with all of your findings. I have an iPhone 4s. Hope they issue a fix soon.
With that "get back" phrase what I'd also suggest is do NOT do another over the air upgrade of a major system like the OS. Especially perhaps with a couple of years old model device where there's often compromises and misfits.