As usual, Radgirl above has it right. Your efforts here should be at clarifying your diagnosis and determining what the course of action should be. I find the delays in finding out, which you have experienced, appalling, and you need to be raising holy hell about that. This is a matter for you and your treating/diagnosing medical people to sort.
After that, then you can get to dealing with the aviation medical people.
Keep in mind that many brain tumors are not cancerous, but still significant due to their location. (Pituitary adenomas come to mind.) And treatment for these has advanced enormously, with excellent results. It is a very sub-specialized area of expertise, and you need to insist on being referred to the proper place. There is every reason to hope for a good outcome here. All the best.
Graham (MD, but not AME!)