I'm not saying that you can account for true love. I'm saying that Thomas could have been properly prepared. He did not have that chance; he was ignorant of his true nature until it happened.
I'm also not saying that his demon wouldn't have disagreed. But Thomas managed to beat his demon for years with nothing but his own willpower, after it had fully grown.
It doesn't matter that true love is rare, or that his demon would've tried to override his instincts, or that kids have hormones and don't want to listen to mom and dad. Him having sex outside a loving relationship has a one-hundred percent chance of turning him into a vampire for life. There was a chance it could've been prevented, and that's a chance that's worth taking, isn't it? I'm not accusing Margaret of not vaccinating her son, I'm accusing her of not telling him that diseases were a thing and leaving him at a leper colony. Maybe there's a good reason for that, and that's what I'm wondering.
As for the last bit, my questions is why she couldn't take him. She was, evidently, able to leave him a pentacle (which I suppose Raith never noticed, or I suspect he would've taken it). Was it an action-movie escape sequence, where she was fleeing into the Nevernever from armed vampires, or was it planned in advance, which might have been adjusted to take Thomas with her? Was she unwilling to take a kid on the run? Or was there another reason?
My point is that the kid was five. You try tearing me away from my kids, and I'll leave you a puddle of unrecognizable fleshsludge on the ground. I can't believe that Margaret didn't have similar instincts.