Rhymenocerous, much thanks for posting your experience, a far more valuable input than my theoretical postulations. The point is it can happen, and if it can it will.
intbh, it is most unlikely that I will be visiting an Airbus Flight Deck any time soon. No amount of realising how unlikely an event it might be gets us very far, for the point is; it can, and has, and can again. As to the armrest v the side stick, here is a diagram (again courtesy of Google) :-
I'm not sure what position the arm rest is at though. Lowest, middle, highest?