The scan is good, and rules it out. MRI sees very well on soft tissue and negative is good news.
I would see a dentist and get a mouth guard for the grinding and work on that. The pressure on the teeth and the muscles can cause immense headaches, so rule that out and work on the grinding. I have worn out several guards myself.
Floaters are a nuisance, but as far as I know benign.
Yes. It is possible as I am living proof. After a car accident in December of 2014, my pain Doc ordered an MRI on my brain to rule out head injury for the headaches (I took two air bags to the head). Those results came back normal, with the exception of some white spots on my frontal lobe, which my PCP told me not to worry about. I won't get into the years of symptoms and the psychiatric therapy I'd put myself in because my PCP had convinced even me that it was all due to Anxiety. Two weeks ago, I was in the ER and a CT scan incidentally located a tumor in my brain. The following week, I underwent another brain MRI - this time with and without contrast. This MRI confirmed the mass, which they believe is a benign meningioma. I was concerned about the size because I believed this thing must have been less than 3 years old, only to learn that the Radiologist compared this MRI to the 2014 MRI, and claimed it WAS there back then and looked "unchanged". So, yes . . . it is possible. I have so many mixed feelings about the years of confusion and misdiagnoses (I was also misdiagnosed with Glaucoma this past October as my Optomotrist had no explanation for the stark decrease in my vision). But I'm glad to finally know that I wasn't crazy. My symptoms were real, and now I know why. Hope you ride your GP better than I did my PCP. But it does happen - at least it did to me.