Being a CRNA is not supposed to be a high stress situation, although it does happen. It's a very controlled environment.
Also, CRNA school doesn't give you any credit really for being a FNP beyond the college of nursing classes you may not have to take. You will be enrolled for 3 years and have to do many thousands of hours of clinical, and are generally not allowed to work while doing so.
If you like the ER and already have a FNP degree, why don't you just work as a NP in emergency medicine? You can get an acute care post-masters with a couple of classes and 500 clinical hours. This would allow you to work ER and take care of all patients. It is a much shorter road and seems closer to the idea of what you want to do.