You are way over-thinking this. This incident happened 7 years ago. You already told me your mom is/was alcoholic, lonely, and maybe had mental health issues. You told her to get away. She did. You said you feel she wouldn’t actually do it. The fact is, she can’t.
So I have to ask myself, why you are thinking about this? Is your relationship with your mother okay now? Do you feel that event is getting in the way of being as close to her as you’d like? If that’s the case, maybe having a heart-to-heart talk with her would help. And maybe not. If she is still alcoholic, narcissistic, impulsive and not too bright, she may not understand how shocked and hurt you were by her suggestion and why it still hurts you now. In that case, your best bet is to find a way to forgive her for doing something impulsive and stupid when you were a teen and move on.
I wish you well.