The way it works for me is that my cardiologist orders the test and the radiologist follows those orders. If you have been prescribed a chemical stress test by your cardio and the radiologist doesn't think that is safe, I think you need to go back to your cardiologist to discuss.
I have always understood that AFib is a (very) rapid heartbeat, not missed beats. I think missed beats are not as serious as AFib but this is another discussion to have with your cardio.
And in the meantime, I would follow direction to not exercise or lift even 25 lbs.
This is a learning process for all of us and I always make a list of issues I don't understand or questions I have.
I wish you good luck with all of this. You will feel better once you have more details.