It doesn't really matter who the official #2 is for Ineos as the idea that anyone on the TdF team outside Bernal is a contender of Yellow is a complete fantasyBrailsford must know more than we do. He'll look silly if Carapaz doesn't have the legs. Or only has the legs in week one when (other) teams can easily control the race.
Could still be that Sivakov is the actual backup plan, but they're not hiding the fact that they're hoping that that guy delivers, either. I'm gonna go ahead and assume that Carapaz is their actual #2.