I don't think there's anything wrong with speculating about it, but IMO, there's no indication at all that would lead me to this conclusion (I do think it would be a dangerous avenue for MSM to take without evidence, but here on WS, I think it's fine). But again, there's nothing on which to base it at this time. It would be like me saying "I bet she was dragged off by an alligator." Nothing at all to indicate that.
I also have to point out that if she was having a psychotic break, it would not be limited to the child on the interstate. Psychosis is a thought disorder. There would be a level of agitation, paranoia, disorganization, and possibly incoherence in her call with 911 and with her family member. People don't have a first break and calmly talk to others about the hallucination. In fact, a hallmark of first break psychosis is (9 times out of 10 auditory hallucinations, not visual), for example, a self-inflicted stab wound or gunshot wound, jumping out a window, or running out into traffic. That's because people don't think rationally during a psychotic break. They lose touch with reality.
Her 911 call will be telling if she was experiencing a mental health crisis.