So I'm pretty late to catch onto this theory BUT,
Isn't it implied that Riku has Terra's heart? Yet I have seen no argument on it.
So why can't the same be for Ven/Roxas?
Anywho,
I believe that you are using the word 'Counterpart' incorrectly.
You're saying that keyblades are like kingdom hearts. Yeah?
Well, During the Keyblade War, the Wielders wanted the power of KH for themselves, yet they already had keyblades.
So if Keyblades were like KH, the wielders would have had a power such as one of KH. Follow?
So if it would be true, Wielders would have the amazing power of Kingdom Hearts, or one like it.
And so far, I have yet to see a Wielder have extraordinary powers of a "higher existance" like Xemnas said he would get from KH.
Which would throw off your Keyblade is like a heart/Gives them emotions part.
Other than that, this theory is pretty solid.
Xemnas' mysteries will hopefully unravel in further KH games.
And I'm not an expert of KH, so if anything I said was false, bare with me, and correct me. Thankyou