What does mildly low-lying cerebellar tonsils mean? What does unremarkable unenhanced brain parenchyma mean?

It depends. Everyone's brain "sags" onto the skull base when standing, running, defecating. Any sag more than 5mm may suggest chiari malformation or low pressure headache. Have your doctor ask the radiologist to measure the mm of descent for you, the rest of the report is normal.