The decision to remove a meningioma is based on size, location, risk of surgery, and symptoms. Therefore without having seen your father in June, I can not comment on whether surgery was an option. Typically larger masses which are causing symptoms would be removed. However, if a patient has other illnesses which makes surgery risky, or the location of the tumor is difficult, then a period of observation may be taken. I hope this answers your question.
Whether it should have been removed in June 2003 is a very tough question indeed, but one thing is for sure: at that time -- in June 2003 -- your father should have at the very least been referred to a neurologist, or more preferably a neurosurgeon. I am very surprised that the ER doctor and the internist didn't have the presence of mind to order the appropriate referral. Since it wasn't emergent at that time, maybe the ER doctor isn't to blame, but the family internist was clearly over his head (beyond his expertise) by attributing possible neurological problems to uncontrolled diabetes and depression. It is good to know that your father is now under the care of a neurosurgeon who knows what he is doing. You can ask him the same question you asked here (why was it dismissed back in June 2003 and whether it should have been removed), but don't be surprised if the answer is less than forthcoming. He will probably say "it's hard to say" or something evasive.
But let me give you a word of advice: in the machinery of modern medicine, it is imperative that you take a proactive role (like you are doing, which is great) in your own care and the care of your loved ones. There is too much -- way too much -- red tape involved everywhere you turn, and oftentimes things do not properly get done and people (lives) fall through the cracks. People get lost in the machine of modern medicine.
Not to shatter anyone's belief in the infallibility of doctors, but everyone should be aware that according to reports published by the Institute of Medicine, JAMA, and the Harvard School of Public Health, anywhere from 44,000 to 180,000 people die each year in America's hospitals due to MEDICAL MISTAKES. Not bad outcomes. Flat-out errors, resulting from miscommunication and carelessness. Two of the biggest culprits are errors in medicine dosages and interactions, and hospital induced infections.
Matter of fact, medical errors are one of the leading causes of death in America today. And these errors are entirely preventable. How about that?
Bottom line: be proactice, be assertive, and don't let the machinery of modern medicine compromise the care of you or your loved ones.