Trying to learn QED...are my assumptions correct?

[sharpstones]

When mass is constant, momentum is a function of velocity and velocity is a function of time. So would it be incorrect to say that the more precise your time measurement (or the smaller timeframe you are looking at) the more uncertainty in the position of the particle a la Heisenberg Uncertainty Principle? And if it is a correct assumption is my line of reasoning correct then?

Moving from there, through Quantum Mechanics it is established that an electron moving from one place to another, has certain probability amplitudes applied to all the possible routes it could take to go from point A to point B.

QED on the other hand, deals specifically with the interactions between light and matter. So because these interactions happen on such a small time scale is this why we cannot disregard such weird probability amplitudes as electrons moving backwards in time, photons moving instantaneously from one spot to another, etc?

I am very much interested in whether my logic is applied in the right direction at all. If there are any assumptions I’m making that are incorrect please tell me.

[Janitor]

Of some help might be this tidbit from Zee's book on field theory:

Did I speak of an electron going backward in time? Did I mumble something about a sea of negative energy electrons? This metaphorical language, when used by brilliant minds, the likes of Dirac and Feynman, was evocative and inspirational, but unfortunately confused generations of physics students and physicists. The presentation given here is in the modern spirit, which seeks to avoid these potentially confusing metaphors.