I think it was always phrased more like 'will have his grace' than 'will gain salvation' in the text, whether or not that's any better. You got the impression it was probably the Hindu equivalent of the same concept, but given his research fail in other areas it would be worth asking whether the Hindus even have a similar concept.

The way it actually played out, I dunno, kinda implied that the two of them figuring out how to get around the conundrum was part of the test, and not killing her turned out to be acceptable? IDK, it's been a long time since I read it, but the whole idea of a couple of morally ambiguous characters getting sorta screwed-over by contradictory instructions from a questionably benevolent god worked pretty well in context. Still, we're talking 'pretty good' for cheesy, angst-ridden, YA vampire romance here. Popcorn entertainment with a few halfway-interesting ideas about religion mixed in for flavour, sure. A soul-changing experience, no.