1 . Japanese CBT didn't need a key so doubt it
2 . Graphics cant even be changed in the Japanese version besides resolution so don't think so.
3 . There's an english wiki for this game easy to google, there's a Japanese one too that's more complete but you'd need to be able to read at least some basic Japanese since google translate doesn't help much there
4 . http://onigiri.wikia.com/wiki/Stats
5 . They'll add up to at least Sekigahara I believe