MMMark, thanks for starting such an interesting thread.
I like you. You seem like a very intelligent, analytical guy who is trying very hard to understand his girlfriend and polyamory. I'm analytical too but my process was much more emotional.
In many ways polyamory defies analysis because each connection is unique. My metamour is deeply spiritual, vegetarian, celibate, artistic, thirteen years older than my partner and long distance. Her relationship with my partner is ten years old and pre-dates ours. My partner is atheist, a confirmed meat-eater, very sexual, and techy. Their only connection is probably recreational (although that doesn't do it justice at all) but she's coming for a visit in a few weeks and he's like a six year old a week before Christmas.
I've given up trying to analysis it. All I know for sure is that real love only makes people better people and we shouldn't be afraid of it.