I don't have schizophrenia so I don't speak from personal experience, but it sounds bad enough that a spinal tap is worth the diagnosis and potential relief.
Sure, but any test that requires something painful like a spinal tap requires someone to already be in a dire state. What I mean is if it could be done through a blood test prevention would be much easier to test for without needing to have a severe episode first.
Approximately half of the people with schizophrenia don’t believe they are ill, which is why they refuse treatment (see: anosognosia). It is a very strange illness.