Not sure if you fixed this yet but I had the same exact problem. Left my switch off for a few months and tried turning it on only for it to die at 80%. I tried doing charge cycles and using the battery desync fix to no avail. I had to open the switch and unplug the battery. Once disconnected I...
I used to change shells on old used phones all the time back in the day. Maybe you can find an OEM shell on ebay or something and pop the internals into it?
Spoof your system to 5.5.1 then download the Pokken update using NewNUSGrabber. Install with WUP installer then you can play without the game trying to update.
Info on how to do that: http://gbatemp.net/threads/wup-installer-title-installer-firmware-spoofing-5-3-2-5-4-0.397053/
@Xdqwerty, to answer your question, they're a fusion Brit-pop/J-pop/electronic band with a woman vocalist. Flamingo is hands down their best known song but they've got a ton of other really good songs