Would an MRI be able to show pituitary cancer?

Perhaps. Mri of the brain with contrast (gado) can show a pituitary tumor but it won't always distinguish cancer from benign tumor. You will need a biopsy (surgical) to fully confirm the diagnosis. Please discuss fully with your neurosurgeon.
Yes. The most common pituitary tumor is an adenoma which is benign. Cancer is very rare in the pituitary gland.