I think when an ensemble cast in a top movie virtually ' claim' roles and make them their own - it is hard afterwards to put someone else in their place. (Other than cases of complete miscasting, none of which apply here.)
That is an interesting list of other contenders and I think most of them would have done really well in the roles they were being considered for, but especially the Addison DeWitt part.
As mentioned, George Sanders was perfect for it and did it beautifully, but looking at the type of sophisticated, vitriolic, suave, urbane, clever, world-weary and cynical soul who was Addison, I also think that Vincent Price and Basil Rathbone would have been excellent.
Perhaps not quite as perfectly but nonetheless effectively, would have been Clifton Webb and Charles Laughton.
Kerry...I am with you, on the Addison Dewitt role on all four of those actors. Out of them...I would think 'the best' would have been..Vincent Price or Clifton Webb...but Laughton and Rathbone would have also been very well cast in this role.
Sanders is lucky to have been chosen, because I see 'this role' (the way it was written) had "OSCAR" written all over it and 'any' of these actors could have also have won the Supporting Oscar if they had been chosen. The 'role' screams...'oscar' from beginning to end!..