On Apple, the file not found error usually occurs because the Apple iCloud management is removing local copies of song files because it doesn't think you are using them. Might be possible to make more storage space on the device to avoid the file not found problem.
Regarding the mixing... that only works with songs that you purchase from iTunes directly. For Apple music subscription songs, Apple does not provide any way to control the volume of the playback without controlling the whole device volume. Mixing without being able to fade the song at start or end sounds pretty horrible.
If the mixing option is important to you, then a reasonable option would be to purchase individually the songs you are using for walkouts mixed with announcements and then use subscription songs for the rest of the app.