I think she did have a way to prevent pregnancies, she just wasn't thinking about it. That's why she panicked the next morning, sort of the way muggle teenagers sometimes do when they realize they forgot the condoms. irresponsible scatterbrained teenagers.

I'm leaning towards the idea that Eileen might not have kept Severus (she never really thought about it much) if she didn't know that it would cause a paradox. The wizarding world has a lovely history of awful things happening when paradoxes occur, according to Hermione, and Eileen is intelligent, even if marks didn't show it.

He looks like an underfed sallow un-bespectacled Potter. yes, this infuriates him, and it's in there partly to torture the characters.