And if we continue the train of thought and suppose that the trainee for some reason (seizure!) accidentally pushed and held the takeover button on their stick - or that their sidestick suffered a malfunction such that their takeover button became permanently pressed - then even in such a case the TRE's sidestick would regain priority once the TRE pushed their 'takeover' button, as the last discrete button press (by which specifically we mean a transition from unpressed to pressed, not a continuation of the pressed state) assigns priority.
Thusly a wedged/locked sidestick with a wedged/locked priority button is still overridden with any single press of the priority button on the other stick at any later point in time.
What could not be overridden, I believe, would be 'rapidfire' of the priority button on the trainee's side.
I could only see that type of input happening - in the remotest of chances - if the relevant wires to the button had become loose or fractured and were intermittently making and breaking contact as the wire moved.
I suspect also this can be ruled out as I would imagine that the sidestick buttons are double-pole-changeover type, with a button press simultaneously closing one circuit and opening another? I imagine this knowing that even the brake pedal interlock switches on economy cars work this way. But I do not know for sure!