Marco having CoC would make no sense. Do you really think that Marco wouldn’t pull out his CoC when the executioners were one second away from chopping off Ace’s head?
There was seven confirmed CoC users at Marineford, there’s certain to be more, and yet not one of them except Luffy used CoC. It’s a case of drama trumping plot- Luffy’s use of CoC is a dramatic highlight in the arc, where near enough every character we knew at that stage commented on how it sets him apart and makes him a huge threat in the future.
The effect of that scene would have been dramatically lowered if everyone on Marineford was firing off their CoC at all cylinders, making Luffy just one of many. And the effect of the scene would have been completely non-existent if Marco had done it instead of Luffy.
You’re also making the assumption that Oda has planned out years in advance all the different aspects of CoC and who will have it, which I very much doubt he did. Maybe at that stage in his planning Marco didn’t have CoC and it
was meant to be really rare.
Now it blatantly isn’t, and if Zoro, Rayleigh, Oden and especially Katakuri, as right hand men can have CoC, there’s no reason Marco or King can’t. Never mind people like Hancock, Doflamingo and Chinjao.