20driver,
Assuming Customs met the aircraft, boarded and spoke to the Captain, they would presumably be informed as to the reason for diversion and that after that they might conclude that it would not be their direct concern as to the exact detail of how the machine met Terra firma. I can't see them being overly challenging about alternative courses of action in the air, surely not their remit. There has been a diversion (happens now and again) and they would just eliminate their own concerns e.g Med divs etc. Put simply, there has been a diversion for whatever reason, the pax / crew are then processed in accordance with procedures by the authorities. Customs might wonder about it all, but 'speak sternly' to the Captain about a decision (right or wrong) to divert or to be angry about it, I'm not convinced about that.