switchbladeeyes

Phew... difficult question. Never really thought about it, because in my mind vampirism was something contagious, like a kind of virus, as Natalie puts it. Or maybe even something like in Anne Rice's vampire lore, a piece of something demonic, that makes the human body mutate and have all the vampire traits and abilities.

There's a hypothesis (don't recall how it's called in English), that says that a religious object can hurt/repell vampires, because other people believe in the holy aspects of said object. That it's not the object itself, but the belief itself, that makes the object a weapon against vampires.
This, along with the "evil is a state of mind - as is vampirism" could lead to Nick being able to tolerate the proximity and even touch of a cross. When he meets Joan of Arc his hand bursts into flame even though he doesn't touch the big cross in the church, and in the episode, where he holds the golden cross on the necklace at the end, he can hold it in his hand for quite some time with minimal damage to his skin.

I'd disregard the "the vampire has a split personality and thus can forget she's a vampire" episode, because... come on... just nope. Even Nick had reactions to food and sunlight when he forgot who he was after being shot in the head. But yeah, good ol' Plot Reasons. ;)

It's not surprising to me that Lacroix said that Nick has goodness in him. After all, he's been trying to get Nick back on the "dark side" and always bemoans that Nick cares way too much for mortals.
Maybe he didn't really believe that Nick was possessed by a demon in SOB, but he could have thought that he needs to encourage Nick, so he can "fight the demon" with this mindset. I bet he would have said about anything to save Nick - like he always did, no matter how Nick ended up in a dire situation.