In case you have recent backups from all your iOS devices from pre February 9th you could try to revert back to those backed upped states (after backing up your current state) and that way you might be able to get the 1.7 iOS app back.
After quite some trial and error I achieved just that. I wrote about it in more detail in this thread: Roon 1.8 - where can I see tags & playlists for a track? - #112 by naja