I think I figured out that this is an issue syncing with iCloud. In the settings for Music, I turned on Show All Music, that includes showing all music, including everything stored in iCloud. Well, all my music showed as being stored in iCloud and needing to be downloaded to my iPad, including the songs that Music said were already on my iPad. So, I tried to download one of those songs. After downloading, I could then delete it from Music by swiping left and having the delete button show up! After that deletion, the song does not show up as being on my iPad. In addition, after downloading songs from iCloud, then the Music app does show up with those songs in the usage>manage storage list of apps. And, the downloaded songs show up as being able to be deleted from there, too, in addition from the Music app. So, I think the list is in kind of a messed up state, with it showing music as being on the iPad (which it may be, since I can listen to those songs) but they are not deletable unless they are first "downloaded" from iCloud.