If I remove her from the auto policy, but she remains the sole owner of the car, does this create any sort of future problem with that car if involved in any sort of accident?
You need to stop dancing around this issue and go to your agent with a LETTER saying:
"My mother has mild dementia. Her driver's license has lapsed and she will never drive again. I have all the keys to the car. Please relay this information to the underwriters and come back and tell me what has to be done with the policy and the car."
Then, whatever the underwriters tell you to do, you do.
Bottom line though, there is no alternative to selling her car, or at least titling it over to you so it can be insured for your use, and dropping her policy.
Whatever else you think you can accomplish with your "what if" questions isn't going to happen and if you do have an accident involving her car, when the claims people find out you concealed your mother's condition from them (and they will) the claim can be denied and the policy rescinded leaving you to pay for any damage or injuries or lawyer fees that the insurance would ordinarily have paid for. That is a nightmare you do not want to experience.
Probably just easier to transfer title and eliminate any possible problem.
That's exactly what you should be doing.
There is no gimmick that will change that necessity.