It would be a plothole if Minato and Kushina knew it was Madara. That's the context.
So your complaint amounts to "It might MAYBE be somebody other than Madara, despite the fact that they can't have known he is the number one suspect, and they are acting exactly how they should have even if it was Madara."
Fine, it COULD be somebody else. The rest of the manga doesn't support the theory, especially since Jiraiya and Itachi went out of their way to confirm, in a narrative sense, that he is Madara.
Let's not forget that you were whining at me for thinking Madara COULD have been Danzou, about this time last year. I had a stronger argument for it then than you do now.