Installing the foam in between the door panel and door will mimic the same affect your finger has, only it will be applying pressure from the backside. Everything has a resonant frequency, at which it will vibrate uncontrollably. By placing your finger or foam at that point in the door panel, you are in essence creating two separate panels that require more inertia to reach the same loudness of vibration you have now. The door panel still resonates, just at a different frequency and hopefully lower amplitude. You will never eliminate all the vibration noise, the goal is to lower it below audible levels. In other words don't overthink the sound deadening, just quiet each rattle as you find it.
That said, my car I'm working on now, I gave the doors the full treatment before even installing the mids.