Help Sanji could contribute in.
The very personal nature of the current fight does:
- Yamato did not say that she is fighting Kaido to help the alliance.
- Yamato did not say that she is fighting Kaido to stall for Luffy.
- Yamato did not say that she is fighting Kaido to open the borders of Wano.
- Yamato did not say that she is fighting Kaido to fulfil Oden's will.
- Yamato did not say that she is fighting Kaido because she's Kozuki Oden.
Yamato is fighting Kaido to sever the bond that binds them. She's fighting Kaido to escape from Kaido's control and dominance over her life. She's fighting for her freedom.
Yamato's confrontation with Kaido here is
deeply personal. It's
the most important battle of her life.
Until she gives up or is defeated, no one — and I mean no one, not even Luffy — has a place in that battle.
Oda stalled Yamato from reaching the Rooftop until after Luffy was defeated. If Yamato was to fight Kaido with someone, it would have been with Luffy.
She may do that in a subsequent round, but this battle in particular is for her.
She'll see it through on her own.
I think Yamato needs to come to trust that Luffy can defeat Kaido, so I don't see her losing before Luffy returns. And even if she did, Zoro has a much stronger case to face Kaido by himself.
If Zoro and Marco are the only people that can handle Kaido, it will be Zoro that Oda will use.
As for how far Yamato will push Kaido in a fight, eh. I don't have strong opinions on Yamato's combat ability beyond that she's significantly below End of Wano Luffy. It's not like we know her ceiling:
- We've never seen her Devil Fruit.
- She's oneshot every opponent she attacked
Oda could make Yamato considerably stronger than Marco if he wanted to, and it wouldn't contradict anything previously established in the story. He could give Yamato the ability to clad her attacks in Haoshoku, and by so doing, unequivocally depict her as a Top Tier:
Yamato will not defeat Kaido, and she wouldn't push him as far as Luffy will in their final battle, but beyond that, she has no clear ceiling to how well she can perform.
I don't think it's necessarily the case that Yamato cannot hold a candle to a serious Kaido.
As for the Zoro portion of you post, I'll just wait and see what Zoro does, says or thinks during/after his recovery. Zoro's intentions will make it clear where he needs to go.