@Daz:
…
I don't engage with "what ifs" because they usually don't grab my interest, unless it's about something really meaningful and unavoidable in the narrative. But if it's about the arbitrarities of fight choreography and character design, I won't be bothered… also because the discussion is endless -- what was done will never be enough because there'll also be other ways it could have gone down.
Besides, readers don't have perfect information either, so we should accept that we can't measure why each little thing in the fight happened the way it did for whatever reason -- it's vague and up to interpretation. As an alternative, what Yoshihiro Togashi does is to write every single thought that goes in his characters' mind during a fight in order to justify every move they make beyond our questioning... but I don't even think this is good writing in general, actually, although Togashi does write many great fights with his style.
As far as I understand it, Big Mom doesn't fight much with Haki. It's not her style. And I praise Oda for this decision because it's much better this way for fights to be interesting. I mean, Haki isn't a visually appealing power system for a comic book -- it's actually invisible, and relies on soft and ambiguous visual effects. Also, Haki isn't a versitile power system either with only a few branches of abilities. All in all, it's as a flawed power system for storytelling in the manga-format, but it needed to exist in order to balance the characters in One Piece -- and, fortunatelly, it has been well done regarding Luffy's progress in the story.
Luckily for us, Haki is not the be-all end-all technique in OP, because it's supposed to be balanced with the other power systems in place, so Oda can do anything by using only the ingredients he wants for each character. And I guess I've already said enough on this subject matter regarding Big Mom.
As for the battle in Onigashima as a whole, there are things which could have been executed a little better, but it's mostly presenting me a great spectacle.