Barbarella has given you some very wise advice and suggestions. Here is my take on your specific questions:
"So my question is...would you believe the doctors or would you go get an MRI and other various tests etc? "
I would certainly believe the doctors. They have extensive training and know what they are talking about. An MRI is an expensive test and the doctor does not think it is necessary in your case - I would go with their recommendations (or lack thereof).
"And could I have had such a bad panic attack like that for over 2-3 hours and not have been hyperventilating that bad even though I was close to passing out? "
YES!
Now, my question to you. What treatments are you getting for you anxiety disorder?
The tests you have had all ruled out a physical illness that can cause anxiety. A brain tumor does not cause anxiety, ask your doctor if in doubt. A collegue of mine died of a brain tumor she did not even know she had, she woke up one morning and was paralyzed on her one side, everybody thought she had a stroke, tests showed she had a brain tumor. If doctors suspect a serious illness they run all the necessary tests, big time.
Blood tests would reveal that if you had a chemical imbalance.
Yes, a severe panic attack can give you the feeling of passing out. I had severe panic attacks without hyperventilating, and with hyperventilating. I had them so bad that I developed agorophobia. I had them so bad that everything went dim/dark, and I almost lost my vision, at times I got blurred vision so bad that I could not read a traffic sign all from anxiety/panic. Or I saw nothing but snow in my vision.
Anxiety can all do that. Your tests proved that there is no physical cause. You have to TRUST your doctors. If you're still not satisfied ask for the MRI if your insurance company covers it or if you can afford it, but I'm telling you if your doctor thought an MRI was necessary he/she would have ordered one.
You need to get help how to overcome and understand your panic attacks. Did your doctor recommend anything? As in seing a therapist?
If you doubt your doctor you need to tell him/her of your concerns, and tell them "why" you doubt them. DON'T EVER LEAVE HIS/HER OFFICE IN DOUBT OR NOT ASKING QUESTIONS. That is what you and your insurance company pay these doctors for which includes answering and addressing ALL your concerns. And if you have no insurance they still get paid, they still have to address your concerns.