As for fonts, you'd want ones for NEC. Like, when you look on saberfont.com, you'll notice the fonts either say optimized for Plecter or NEC. You'll want the ones for NEC; the Plecter optimized fonts won't work as well, because the sound files are generally in different formats with other settings.
To change/load/edit the SD card, you will need to download the spark configurator file thingy off their website.
Champion sabers do NOT have an editable soundfont. They are simply Viridium or Crimson.
The quick connects on the LEDS are plugs. They're not a structural weakness... instead of having a solid wire from board to LED, they're wired into plugs, which connect the two, making it easy to unplug the LED and swap it for a different one. So no, I've never heard of them falling off, or coming undone. Even if you were to vibrate the hilt somehow hard enough to knock lose the connection without doing other damage to the internals, all you'd have to do is slide the LED out and plug it back in. Which literally takes about 10 seconds. lol