1. I honestly don't know precisely about this point, because I just got a Libra and I didn't experience any software updates yet. Your UltimateMangaReader should work, PBChess worked with me in my Libra. I might as well try it, you made me discover another great app!
2. They *could*, but in fact, all you'd have to do to keep your warranty is doing a hard factory reset if there is a problem. These can be initiated by physical buttons, and if (maybe) your screen doesn't work anymore or a thing like that, you could still restore it to its original firmware.
3. Not begginer tips, but here's the software I installed on my Libra:
1. PBChess with custom apps like
https://www.mobileread.com/forums/sh...d.php?t=296647 which are very cool + Tazuku
https://www.mobileread.com/forums/sh...d.php?t=224906 for Kobo Launcher.
2. Plato and KOReader for reading PDFs and downloading news
3. InkVT
https://github.com/llandsmeer/inkvt which is an awesome terminal emulator for Kobo devices, Python3, NiLuJe's stuff, etc.
4. Some software hacks (landscape orientation system-wide, screensaver pictures hack, devmodeon hack, which enables a fair amount of games in the 'Beta Features' menu, etc.
Good luck, and be careful not to break it while hacking!