Your power button should not be redirecting to other functions. Be sure that the latest firmware is installed, it solves two issues: the LOCK button is retasked to the power button, and the latest version solves the irritating "dead volume controls when locked’ issue.
I have used embedded art and separate “album art.jpg” and “folder.jpg” successfully. I’m spoiled, as I use either Windows Media Player or Rhapsody to write the files in MTP mode, where there’s a separate “album” folder with art. This is one of the little things that MTP automation provides.
It shouldn’t be a problem to manually use MSC mode, though, since I’ve transferred several podcasts and added art while being plugged in using MSC (at the time, restarting a comm session in the other mode wasn’t worth the hassle, I just had to rethink my methods a bit).
Personally, I’d install the latest firmware, and then start anew with a freshly-formatted device. You shouldn’t be experiencing odd control responses. The main issue I note is that the volume buttons were problematic.
A brief footnote- I have a Fuze+ with a little quirk, the Power button is a wee bit sensitive, due to the position of the internal PCB. It’s well within tolerance, but a soft “tap” of the device against my trusty cushion, a neoprene mouse pad, makes a notable improvement, as the button’s travel improves ever so slightly. With the Fuze+ aligned vertically, I tap the bottom (headphone connector edge). It’s the only one I’ve seen with this issue, but it’s always operated fine.
Be sure to have backups of your music, in case of disaster.
Speaking of disasters, I had the opposite, a crashed computer following my daughter’s sortie into some gaming sites. She didn’t let Daddy know about the “firewall OFF” flag that had been up for 24 hours prior to the crash. I used the Sansa in reverse, reloading the music library from flash memory.
Let me know of your progress, and if the album art resumes normal operation.
Bob