I wrote a long explanation somewhere else on here, but I can't find it at the moment. Anyway, the short version is that it isn't your low performance in medical school that is going to be the hindrance, but financial concerns.
The government subsidizes all resident education in the US, and it's not a small subsidy. Resident education is not cheap. When you signed up for FP residency, the gov't allocated three years of money to subsidize your education. After 3 years in FP residency, you've used up all your subsidy. Any program that takes you now will not receive any gov't subsidy for your education. You'll be competing against candidates who will receive the full subsidy for their time there, so the program has to be willing to lose that money to take you.
Programs willing to do this are not common to say the least. Academic programs already pay their docs a fair amount less than what they could make in the private world. Any subsidy they forego will essentially come out of their pockets.
Now, I'm not a residency director, but this is how it was explained to me by my residency director when I was assistant chief resident at my program a few years ago. I doubt much has changed in that time.
So while not impossible, it'll be difficult for you. Please PM me for more info.
I agree that you could probably handle most of what comes into the ER, because honestly the majority of what we do is primary care. But the philosophy of emergency medicine is different, and it's plain whenever I talk to admitting physcians from various specialties. Because of that difference in philosophy, non-EM trained physicians will miss a lot more than we do. Those misses will cost, sometimes a lot.
If your work in your FP residency was exemplary, then your medical school record really shouldn't be an issue. Medical school records are important for new grads because residency program directors have no other data to judge whether or not they'd be good residents. You presumably have already proven that in your present work, so the medical school record would be of minimal importance unless you were grievously stupid as a student and killed lots of patients. That seems unlikely if you've done well in your residency.
Again, PM me.