Situation just gets worse and worse.
Going back to the plane crash, Cardiff's insurers probably have in their policy that the player cannot do certain dangerous activities (even motor bike riding is forbidden in some policies, something agents should be aware of), which is pretty standard so McKay the agent should have known that, and yet the son arranged the flight in an aircraft with a pilot who was not authorised for such a flight, and in a plane not authorised to take 'commercial' flights (so almost certain to invalidate the player insurance) - although I am sure they will say they didn't know that. However ignorance really is no defence. So imo the agent and his son have some significant amount of responsibility - and that should require them to pay something significant.
Cardiff City are arguing technicalities but if their argument is only that the paperwork was not in the correct form to be registered at PL, whilst they may be legally correct if the contract between Nantes and Cardiff had a condition before completion could happen that the player should be registered with PL, the fact that it was Cardiff's responsibility for the registration to happen and they had accepted him and had arranged for him to be available the day plane crash, makes their stance very difficult morally and possibly legally.
Nantes may be in no real position to pay anything (and has an obligation to pay another French club who trained the player). However just as Spurs found when we sold Crouch to Portsmouth and Portsmouth went bust, the 'sell on' clause on Crouch became useless, and I would suggest that Nantes need to tell the other French club that the amount they will get will be significantly less than they might have expected.
However with the money in the game, I think for the good public image of the game, the PL should broker a deal with Cardiff whereby PL chip in something substantial (maybe even half) and Cardiiff pay to Nantes what is owed. Its fair to help Cardiff out over what is a significant sum (for them), but Cardiff should pay something significant as just as when they buy a player who turns out to be much less good than they thought, they pay. And PL should lean on McKay's to make a contribution - big agents such as McKay make multiple millions out of the game each year, so should put something back when they screw up even inadvertently (assuming they didn't know about the pilot and plane).
Given the money in the game, and the PFA can afford to pay its CEO £2m pa (or whatever it is), it seems reasonable for them (and their insurers if the PFA can lean in them) to pay some compensation to the family of what should have been their latest member - to look after players and their families is their 'raison d'etre). And that money of course should be augmented by the amount pad by McKay (the agent) and his son.
So it just needs a combination of PL and PFA to step up and broker deals - the PFA looking after the family and PL looking after the clubs. But Cardiff and McKay(s) should be made to pay significant sums