I mean, there’s a difference between a schoolgirl who’s been fairly sheltered all her life (yes, awful family, but still) that he’s just seen playing a childish game with her friends and one he met first as a mysterious sorcerer on the run who’s experienced tragedy after tragedy and moved all over the place with a conman for a father and who’s now the most wanted criminal in the city. Plus, she’s audacious and we know Oliver enjoys that:
Oliver turned to look at her fully, his eyebrows lifting. He was disappointed to see no mocking self-awareness in her eyes, and not even a hint of real audacity. No, she had opened the conversation with a trite one-liner, probably memorized and used on any man she found attractive
Ah, I’ve found the bit where he considers the proposal:
He was surprised that they would consider him a viable match for a young girl from such a prestigious background, as a non-Crown Family member, and a foreigner to boot. He was wealthy, true, but marrying into the Gervin Family would be a huge boost to his social standing.
An inappropriately large boost, in the eyes of many.
Margaret was probably only sounding out his feelings on the matter. It seemed ludicrous that the Gervins would consider him a serious candidate.
That thought was reassuring. The other students entering the University were even younger than Siobhan, and no matter how advantageous it might be, the thought of tying himself to someone he didn’t respect, for life, was enough to make his clothes feel too tight and his skin prickle.
She’s younger than Siobhan and he doesn’t respect her (or thinks he wouldn’t).
As for life expectancy…I have a couple of thoughts on this. Firstly, she’s likely to outlive any of her romantic partners if we just go by how things are at the moment. Myrddin lived several hundred years longer than other thaumaturges of his time. She might find other loves after her first dies, or she might just dedicate herself fully to magic.
The other possibility is that she works out a way to extend the lives of others. After all, most of growing old is because your cells start to create faulty copies over time. If you happened to be living with the greatest thaumaturge of all time and she happened to make a replica of your body at its prime, down to the smallest details and “reset” your cells back to that template and healed away the damage - yes, Nulls are resistant to magic but it does still work on them, so she’d have to pour some power into it - I think she could extend his life as long as hers.
After all, look at what she did with Bear and everyone thought he was a hopeless case. She added years onto his life.