Just to point out something, I've got a UK PAL edition (SLES-02442, in case anybody wanted to know) and I've only ever gotten Nanami to survive if I used the Forgiver Sign spell on Gorudo.
Admittedly I haven't played in a few years, but I remember testing it when I was younger. Armour off, armour on. Forgiver Sign, not using it. Saying something, not saying something.
I just remember all signs pointing to using the final spell on Gorudo. Just thinking that it's important is all.
I mean, from a gameplay point of view, I can see that the spell's name shows that, despite Gorudo was an utter arse about killing trying to kill Riou/Jowy and hitting Nanami instead, Riou forgives him which shows the compassion, possibly invoking the rune to aid his distress.
Of course that theory is probably utter crap, but hey, it kinda works.