Hello again:

(1) as far as the wobbly Vader helmet in EMPIRE after he chops off Luke's hand, I would lock the helmet down. Remember earlier they show a machine LOCK his helmet into place when we see the back of his head. This would mean his "outer helmet" is locked onto the rest of the face mask, which would make it stable and not move in the wind.

(2) the helmets worn by the imperial soldiers in JEDI were worn loosely to begin with, thus would wobble. I would still consider locking the helmets down, as this further contributes to the "goofy" and the "look, if you're not going to take this seriously, then I'm outta here" vibe that JEDI gives off so well (which is why JEDI will be a bitch to fix).