The covers are about the straw hats and the 1000th chapter. It's likely Oda wanted to make a callback to Luffy and his dream to highlight his progress 1000 chapters on. Hence he included a flashback of his dead brother affirming his belief in Luffy and highlighting the similarities between him and Roger. Yamao therefore acts as a decent enough excuse to bring up both Ace and Roger for the special chapter 1000 moment.
That's why she's separated from the straw hats in the covers and instead paired in the backdrop with Ace, since that's the only reason she's there.
IMO Both vol 91 and 88 make stronger arguments for their respective candidates as they are actually positioned in ways directly comparable to all the other straw hats. And of course the mere fact that all these different covers exist takes away whatever sway each of them individually may have had as proof for joining.
That's why she's separated from the straw hats in the covers and instead paired in the backdrop with Ace, since that's the only reason she's there.