Hi CS, it's not that important you know when your AS's drinking morphed into alcoholism because it's the current situation that counts. The history leading up to it is something she could explore in therapy if she had a willing mind.
I am a recovered A, and my drinking progressed from almost non-existant in my 20s, to heavier in my 30s and 40s and way too much in early 50s. I can't go back to my moderate days, ever, because the wiring in my brain has permanently altered and I only need to start drinking regularly for it to escalate again. My point being that once you've reached unhealthy levels moderation isn't an option.
I have an alcoholic sister who caused a lot of damage to herself, her home life and her marriage and is paralysed with guilt, but nothing we could say at the time penetrated. She would even deny she'd been drinking when she could hardly stand. I found her addiction doctors, talked to her, there was an intervention of sorts....nothing worked until she met a new guy. She's still drinking too much, but it's much more controlled through her circumstances.
You say you're wary of the religious component of Al-anon, but it's the place where you'll find other people in your situation who know what you're going through. I'm an atheist myself, but I wouldn't hesitate to use AA or Al-anon if it helped. Maybe you can do some exploratory visits to Al-Anon and if it helps reassure your mother that she won't have to sign up for church every Sunday. It sounds like she needs it (Al-A not church).