It's impossible to say what was going on with Amanda based on the information released to the public. It looks like she experienced some psychosis, which was complicated by drugs, media attention, impulsivity, and stress. There are many diagnoses that could be given for what she went through, so we'll never know what her assessments showed.
Going to the hospital for a mental health evaluation before going to jail is standard practice for someone who demonstrates any level of suicidality or inability to care for self (probably what Amanda exhibited), as is a 72-hour hold. Her parents seeking a temporary conservatorship is also pretty standard. I guess my point is that the care that Amanda received is (hopefully) what any allegedly mentally ill person would receive; hers was just more publicized. This wasn't an extreme reaction to the acuity of her symptoms - just standard care.
"There can be no other explanation for how anyone would behave in such an irrational manner." There could be many explanations, and it's complicated by her upbringing and drug use.
Saying it
must
be schizophrenia or bipolar disorder without all the facts serves to perpetuate negative cultural and media stereotypes around mental illness. Did/Does Amanda have mental health struggles? Sure. Do we all? Most of us.
Sorry for the wall of text, and I'm really not trying to be a jerk. It just pushes a button of mine when misinformation about mental illness, and particularly schizophrenia, gets tossed around.