My mom is experiencing mental health issues and is not acting herself. What are my options?

Get her to a doctor, check up for a urinary tract infection first. Hurry.
Report as a change in mental status, urgent.
Unless she has always been this way.
UTI.

In California, you "Baker Act" her. That's a 72 hour psych evaluation.

Will she allow you to accompany her to the doctor? You can fax tje doctor your concerns on advance. This might be as simple as a uti.

Erratic isn't incompetent. Even if she was declared incompetent, the spouse would be the decision maker. You could suggest assisted living, take them to see places, but it is your parents' decision. Is there a POA in place? Do you have all the paper work in place, will, POA, MPOA, and what health decisions you want made, I can't think of the name. You will need this for both parents. Consult your county council on aging, every county has one. Make an appointment, and find out what is available, and what your options are. You are very smart to see the hand writing on the wall..