I think your point about it being akin to Merope Gaunt is interesting, but Tom Riddle snr was an extremely good-looking man. I'm being shallow here, but Tobias Snape does not appear to be good-looking. Severus Snape resembled him (Ha, I am mean) and Snape is never described as good looking. My theory about Eileen and Tobias, is that she got pregnant so they had to marry.If her Muggle husband "didn't like anything much", I still don't know why she would marry him, but a physical attraction could have been the cause, and maybe even love potion...(?).
Regarding Snape joining the Order. JKR has said that he did only join because of Lily. She said that if he hadn't been in love with her then he would have never turned away from the Dark Arts. I do like Vorona's theory about him allying himself with Dumbledore out of self-interest though. He's a survivor above all.
Whilst I think Snape could well have been Sorted into Gryffindor, he clearly wanted Slytherin - he said that on the train. The Hat takes your choices into account - we know that. Why would it place him elsewhere. And we all know that not all Slyths are bad...