Being a caregiver for a parent with schizophrenia is especially challenging for all the reasons you shared. It can be considerably harder when that parent isn't taking medications and, based
on your description of your mom, it doesn't sound to me like she is doing so.
I am in agreement with you and think that it's time to explore whether your mom would meet the criteria for having a guardian/conservator (i.e. someone to manage her financial and medical affairs). However, this usually requires a psychiatrist's evaluation and I am assuming she would be opposed. If that is correct, I would suggest the following two steps:
1)Put everything that you've shared in your question here about your mom's behavior and your concerns in writing and send it via fax to each of her doctors and keep a record. This is valuable information and they should act on it.
Many times family members feel powerless because their loved one doesn't want the communication to take place and there is much to be said for respecting those boundaries in situations where there is no risk to a person's health and/or well-being. However, in your mom's case just the opposite is true and I think that it's necessary for you to intervene. Remember that the doctor(s) may not feel comfortable sharing information with you about your mom because of the HIPAA law, but there is nothing to stop you from sharing information with them.
I would do this soon and indicate your interest in having her competency evaluated, then see what response you get. He or she may be willing to make a referral to a psychiatrist and if you're lucky, maybe even work with you on how to roll this out to your mom (i.e. perhaps the doctor would be willing to leave you out of it entirely and say that it is his/her idea for X reason). If your mom trusts this person, this may work. If you don't get a response from her doctor(s)or the response is not sufficient, I would suggest proceeding to step #2.
2) Call Adult Protective Services [APS] in your mother's home town. This can be an anonymous call if you'd like, but most important is again that you share your concerns and the behaviors you're seeing as well as her diagnosis. In some states, APS is able to send a psychiatrist to the home to evaluate competency and if she were to be declared incompetent, the agency would start the process of guardianship/conservatorship then. My fingers will be crossed that your mom's state is one of those, but you won't know until you ask.
Beyond these suggestions for your mom, I think a support group for you may be a very helpful thing to find. One thing I can promise you is that you're not the only daughter caring for a mentally ill mother; support does exist. The National Alliance on Mental Illness is a great resource for this. Their web address is: www.nami.org.
All the best.