I just published the design doc on github. Key bits below. Let's begin where we left off in the requirements -- with the challenging use-cases, and how the design satisfies those requirements.

Use-case: be familiar - the device should immediately seem familiar to the end-user. 
Design: vintage radio on the outside, MP3 player on the inside.

Use-case: update set of music e.g. adding albums - must be elderly-caregiver-friendly.
Design: music is stored in albums/folders on a regular USB thumb drive.

Use-case: play key music formats - support MP3, AAC/iTunes, FLAC. 
Design: VLC (VLC-NOX specifically) handles music playback, so all key formats supported.

Use-case: create playlist - create a playlist in some sensible order e.g. alphabetical by album.
Design: Python code scans the USB thumb drive, orders found music by album/folder name, and then hands this off as a single playlist to VLC-NOX.

Use-case: have good audio quality - shouldn't suck.
Design: a firmware update from the Pi Foundation that dramatically improves audio quality.

Use-case: support sudden shutdown - no issues if unplugged or if power is lost.
Design: read-only micro-SD card - TMP_WRITE_PROTECT bit set on micro-SD card.

I will write more project logs. I'm not sure which one will be next. Might be about the most convenient way to enable TMP_WRITE_PROTECT on micro-SD cards.

Best,
Ross