Nathan - sounds like you really learned a lot down there, and battled some mungus fish too. I think you are 100% correct that motion is key, or maybe more specifically behavior. In the case of sand eels and crabs for stripers, it's the behavior of subtle movement on the sand that counts so much. The marabou abdomen in my juvie pattern is the essential element that adds a tremendous amount of movement both in terms of the material itself and in the foil created behind it and how it moves the hackles. Clousers are so effective because of the added action (ie. jig).
I hooked a gnarly chrome-bright steelhead on the Skagit River the other day on a wild looking fly called the "intruder", an invention of Ed Ward ("steelhead bum") of Washington State's Cascade Mountain river systems. During summer I tend to imitate, but this was an act of "stimulate" due to the action and movement in the fly.
I think in the end it's the concert of imitation, stimulation, an appearance that suggests familiar food, and triggering motion - all rolled up into a single fly. When you figure that one out please let me know