I'm not really sure what you expect an MRI to show. You've already had one that was clear so why would you suddenly expect to find something now? It would stand to reason that this symptom is related to your original psychosis which you know wasn't caused by a brain tumour... it's not like they could have missed it.

Also just because you don't 'feel' psychotic doesn't mean you can't have hallucinations related to a psychotic disorder... I understand you aren't diagnosed with anything but that doesn't mean your psychosis isn't related to mental health problems. Many psychiatrists are unwilling to put labels on people where it isn't definitive.

I seriously doubt they will be open to the idea of a second MRI since they've already given you one too.