Is there any legal way to force my mother to be held on an involuntary 51/50 evaluation hold, or something similar in California?

Next time she falls call 911. Tell them that she is unsafe at home and you can no longer care for her properly.

If she is of sound mind she can make the decisions to not go to her doctor for follow up visits. If she is of sound mind she can decide not to eat, she can decide that she does not want to take her medication if she has any.
Unless you have her declared incompetent you can not "force" her to do anything.
If she is declared incompetent you or other family member could become her Guardian or if you do not want to take this role then the State would appoint a Guardian.

You could consult an Elder Care Attorney for more help and how to deal with the situation. Many will provide a free consultation. You could also check your local Senior Services or Adult Protection Service to see if they have other ways to intervene.

Nothing you’ve said qualifies her for a 5150 hold in California. The police won’t take her in for being an alcoholic. You can call 911 but the hospitals won’t force her to stay. not unless she’s declared incompetent and under guardianship.

Guardianship is about the only way to “force” anything on her. I’m so sorry, I really sympathize with you. Alcoholism runs in my family and I know firsthand that......there is little you can do for someone who doesn’t want to help themselves.

You know, to be frank, this is what my own MIL did. She didn't live in our state. She was always very narcissistic, the kind of woman that would threaten suicide, not meaning it at all, when her son, my husband was growing up. Very self centered all her life. A woman who had once been the belle of the ball, beautiful and charismatic, the star of her small town. She ended up being severely alcoholic and it was Vodka pretty much morning to night. Had a few neighbors who would bring her alcohol, and she got it delivered. She lived in her beloved desert home, getting a smaller and smaller world until she was down to running the hose a bit and laying in bed watching the wildlife come to drink. She had a few hired caregivers. Was diabetic and took care of her own insulin. Son visited and when it was clear he could do nothing he took to making phone calls every Sunday, hoping to get her more rather than less sober. She died, alcoholic, in her own home, in her own bed. To be frank, it's an option. What can you promise her when she gives up her beloved booze? Likely her body cannot come back from it. The simple truth is, if she is not demented, you have ZERO power in this situation. She has made her choice. Try to know that not everything can be fixed, and try to offer what support you are able, then step away and let her decide for herself what her own life will be for the rest of it. She almost certain will fall and break something. My MIL did. Went through all the viciousness of DTs, rehab, and back to her little desert home and her beloved bottles of vodka.