With a diagnosis of P.O.T.S. and if the fainting happened with standing, as can happen to some with P.O.T.S., then it is unlikely you will pass out while sitting in a car, because standing intolerance is how your body is manifesting the dysfunction of your autonomic nervous system.
I'm glad your echocardiogram was fine. Did you get any event holter monitoring done or just resting EKG/ECGs as far as taking a look at your heart rhythm goes? I think holter monitoring would be helpful in your making a decision about your driving.
How I'm dealing with my own P.O.T.S. currently is trying to get holter monitors every couple of years to make sure I'm okay still to not take any medicine for it... I don't tend to have bradycardia like you did with the slow heart rate, and I haven't passed out except when it was induced by nitroglycerin on a tilt table test. I also have I.S.T., which means my heart can race when resting.
Chest pains, from what I was told by a relative of mine, can be normal with P.O.T.S., which was a comfort to know, but you should (if you havent' yet) make sure iron deficiency is contributing to the chest pain and I think you probably should also ask for a nuclear medicine stress/rest test, if you can tolerate it, to further check on your heart.
Do you have normal lipid panels when they check your cholesterol?
I don't have all the answers, but I can offer some! I have POTS and dysautonomia and a slew of other labels! I have fast heart rate, though. I did have the chest pain as you said. Dysautonomia can cause pains like that. It did to me more times than I could ever conceive!
I have a friend who has a pacemaker and a defibrillator and the doctors all agreed he was safe to drive. (I don't know how I agree with it!) But a pacemaker simply steps in when your heart rate is too slow or uneven to pace it more regularly. The defibrillator is the one that shocks you to the ground when you have a rapid, crazy and unsteady heart rate. It gets you back to a normal rhythm.
I do not know how serious bradycardia is, b/c I'm opposite. :/