Snape would have joined the deatheaters no matter what, otherwise Lily would have persuaded him to give it up at school. She tried over the years, we saw that in Snapes memories. He didn't even give it up when she told him, she had had enough.
Lily and Snape could have ended up together, JK said so in the Bloomsbury chat. But only if he had given up on Voldemort.
Snape as a father? That's a tough one. I suppose if he had been softened up enough to give up his dark dreams he might have been able to make a good father.