If Sanji is nobility, then what exactly makes him any different to Doflamingo who is a Celestial Dragon, yet he's lost all privilleges? Sabo is a pirate who was a noble too, his nobility is of no value anymore, so why Sanji would be any diferent to Sabo or Joker who are fallen nobles/world nobles doesn't make any sense imo.
The fact that they never even had Sanji as "wanted alive only" prior to a good visual of his face despite his wanted poster always saying "Sanji" indicates Sanji might not be his original name since he was only recognized by his face by someone within the WG; perhaps, it was one given to him by someone or one he made up himself.
Except Sanji knows something equally as important as the National Treasure if not more so and somewhat giving Doflamingo a prerogative, then that could explain why he's wanted only alive or he simply possesses information that the WG know he has without a doubt and need it at all costs; hostages or terrorists are only kept alive if they possess information.
The Big Mom wanting Sanji theory too is rather farfetche too; once the WG cater to the demand of one Yonko just to avoid war, is that any different to Kaido requesting for Joker despite his imprisonment? or is that any different to the WG handing Ace over to WB just to avoid a war? No, so the Big Mom Sanji cook theory is rather questionable.