Does the complement rule apply here?

In the following examples, I understand that because what comes after the comma is a complement to what precedes it, the sentences are not considered comma splices.

"It's a tricky question to answer, I know."

"She's at the top of her class, I believe."

I always thought that the way to tell if the latter part is a complement of the former was to reverse it (dropping the comma altogether) and seeing if it still makes sense:

"I know it's a tricky question to answer."

"I believe she's at the top of her class."

All good there. But what about with phrases such as "doesn't it"?

"It goes to show that one day makes all the difference, doesn't it?"

It sounds as though it should fit the complement rule, but if we reverse it, we get this:

"Doesn't it it goes to show that one day makes all the difference." — clearly nonsense.

So, basically, what I'm asking is whether "doesn't it" can be set off by a comma from the rest of the sentence as a complement or if that would be considered an error.

I suppose the same would apply to any sentence that ends with "isn't it", too.