Re: Detaining pax on board - Legalities?
qfcabin as I see it this is a bit easier to answer than may initially appear. The key is that it appears that it was the German (airport) authorities that denied access due to non-available facilities (no customs, etc. and you did not mention immigration which probably would not have been available if customs were not available). The argument is then between the airport and government authorities and any pax that want to leave the aircraft. (In fact airports do from time to time deny access to terminal buildings, especially in the case of multiple diversions where the facilities are overwhelmned. The justification for substantial delays is often safety, due to congestion in the building). The key to such a situation is the role of the National authorities and any safety arguments that might apply.
That being said, the same principle of "detaining the unwilling" arises and one can see a situation where an individual could insist on getting off and various fun and games starting. At a minimum it could be forecast that (a) they would need a good reason (not just convenience) to sustain their argument and (b) they might get off, but they would not be processed through the airport in the absence of the necessary facilities - leading, at a minimum, to a stay in the transit lounge. The consequences of that could be probelmmatic for everyone else ... but would be no different than removing a passenger for medical reasons ... What about their bags .... and the lack of unloading facilities... but that this a different matter.
P.S. RAT 5 thanks for your fine and humble apology ... which was unnecessary. Just setting the record straight.