Would Ben have had a chance with Alison?

Not really, she was completely focused on her career and he was a loser before he got he's shit together to take care of the baby, i doubt him wanting to be with have would have that same effect.

Only if he could bang

Gonna go with a hard no. I'm not one of the people that think Katherine Heigl with way out of Seth Rogen's league either. I think he's cuter than he gets credit for and she is not as good looking as most people think at least imo. But he was a total loser in the movie, nice guy but still a loser. He had no job, was pretty classless, and just plain irresponsible. While she was super focused on her job and just seems to have a holier-than-thou attitude. So I just don't see it ever working. Maybe if they were somehow forced together in a different way.

Yup

If they were in a situation where they saw each other frequently, like at work or something, then yeah, I think so. Because she seemed to like him right off the bat. And that was before she even had her first drink.