Actually you are right your method was correct to get the normal ending. How to get the extra voice you can find in jjewels website actually https://sites.google.com/site/otomeundercover/game-guides/storm-lover-kai#TOC-Tips-Tricks
So it is not custom firmware then? if the version is 6.61 without any letters then it means it is official firmware, if that's so you need to get custom firmware...
The tutorial is really easy here https://sites.google.com/site/otomeundercover/patching-instructions Do you have original umd game 12lifetime? If yes to make iso there is a few ways, do you have custom firmware?