It doesn't sound like you have afib to me. The heart rate with afib is regularly irregular. That is, the rate varies all over the place second to second, minute to minute. It doesn't remain constant for even a few seconds (it's random), whereas NSR doesn't vary by more than 10% over a minute. When I'm in afib and at rest, I might get 4 beats in 5 seconds, 8 the next 5 seconds, 5 the next etc. Over minutes, the rate varies from 55 to 75 (in my case). What you are describing is a increasing rate (due to oxygen debt or anxiety) with time or a high constant rate (SVT?). I would suspect anxiety in your case. You are probably worried about nothing.
Please learn to trust your own doctor, or find one that you feel more comfortable with. Mild MVP with a "trivial" leak is a very common finding, one that a lot of people have and don't even know about. It should not cause you any symptoms. Sounds like you've had a good medical workup and have been given the go-ahead to exercise. Perhaps you should talk to your doctor about anxiety. It seems that you worry somewhat excessively....?? Just my take. Hope you're feeling better.