Welcome!!!
I don't have any first-hand experience of being the parent of an addict, but I have a good friend whose son is going through rehab for drug abuse, and I talk a lot to her.
I think everyone does things that we can pull up and wonder if it was what caused our kids to do this or that. Your son is right, though, an alcoholic doesn't need a reason. And whatever they say was the reason they started, it's in the past and it doesn't matter: What matters is what happens from here on out.
I think the biggest fear my friend has with her son is doing something wrong so that he starts using again. Even though she's working on her own recovery, going to Al-Anon, I think that's really hard to let go of when it's your child you're talking about. You want to help and you want to not harm. And the thought that "it really doesn't matter what I do, what matters is what she decides to do" -- I think that's more difficult for a parent of an addict than for the spouse of one.
I would really, really encourage you to try Al-Anon. The group I go to has people who work the program and are fluent in 12-step language -- but also people who just come to the meetings to not feel so alone. A place where they can feel "normal"... People who don't work a program but just like coming there to be surrounded by other people who deal with what they deal with.
As for your daughter? I've no experience of being an addict. When I've had friends who have gone through rehab, I have basically let them know once they get out that I'm there, that I want to support their recovery, but that I'm not sure what they need and that I won't push myself on them but that they can feel free to contact me when and if they want to.
A lot of people in early recovery are extremely focused on their own recovery. Don't take that personally.