We turned up at a 747-200 to fly HKG-KWI, swarms of engineers around aircraft changing mainwheel tyre in the dark. Work complete, F/E does walkaround (in the dark), off we go- one main leg won't retract (going out through the gap). Much raising/lowering- all tricks, no budging. Landed back after 1 hour, no jettison. The engineers had put a piece of wire coathanger in the maingear lock which the F/E just did not see 12' above ground, buried in undercarriage bay in the dark, with bright lights all around the airport. Result, one severely chastised F/E within inches of his job and well shouted at, with books thrown at head. Wrong of the ground engineers, but ultimately undeniable that F/E was responsible. Very sad, but it happens.
As for continuing to next convenient destination on way home, not on. The gear and doors being down means noise, discomfort, and no real way of knowing fuel requirement, fuel useage etc.- FMS does not compute gear down cruise. Also, will staff be available at next stop? They will probably have gone home and had a drink. It's actually cheaper and easier to land back, sort the problem out at your airlines terminal than try and make next stop, wherever that may be.