This is the account I was given and I could give you a much better one. With much more… accurate answers but she wouldn’t like them.

Yes he has been working at Hogwarts for fourteen years, give or take. When he took care of his sister’s child Dumbledore offered him a more stable job as a teacher. Rather than a Death Eater. When he started, he wanted to do Defence against the Dark Arts but, as I understand it Professor Dumbledore wanted to get him away from the world of Dark Arts, that he had previously been used to. Perhaps to change his mind set. He placed him as Potions Master. Prior background? I don’t know what you mean ;D