TLDR: travelling to Japan and back from Australia with expired Spanish passport (AUS visa linked to it) and valid French passport (AUS visa not linked to it but AUS Immigration is aware of dual nationality info).
My son has dual citizenship Spain and France, with the same name (ie K--- ) and last name (ie M----) on both passports but one additional last name added on the Spanish passport (ie M---- + A---). We have an Australian PR visa linked to the Spanish passport and want to travel for a short trip from Australia to Japan and back.
The problem is, the Spanish passport is expired and we cannot get it renewed (Spanish administration sucks big time in general, no embassy near us to renew it within the timeframe).
Australian Immigration says they can update visa details to the French information (and dual nationality is OK) but that visa might still be linked to Spain when shown to airlines and essentially might cause trouble when travelling, which I don't understand why.
What should I do and how do I travel to Japan and back given the information? I'm thinking of bringing both passports and let the airlines sort it out but I'm scared of what might happen.
Any advice?