Wikipedia states that George Romney’s father did not relinquish his US citizenship. Do you have a source that says that he did so?
If grandma Romney left the US vowing never to return, but did not officially (whatever that means in the context of citizenship) renounce his citizenship, then he would have remained a US citizen. This means that George Romney was a citizen but not natural born, and therefore Mitt Romney, if he was born in the US, was a natural born citizen.
If George’s father renounced his US citizenship in a legally valid way, and George Romney never took on American citizenship, then Mitt Romney would be in exactly the same situation as Barack Obama and, as far as I can see, would be ineligible to be President.
The issue turns on whether George Romney’s father renounced his US citizenship in a way that was legally valid and was not a US citizen when George was born in Mexico.
There is not a single shred of evidence that George Romney's father EVER renounced his US citizenship. I think it is irresponsible to suggest that he did and then demand proof of a negative.
Moreover, it is my understanding that the various colonias that were established in Mexico during the 19th century by dissident religions and political sects from the Unties States were established under Mexico's direct edict that the residents of the colonias, and the children born there, would never become Mexican nationals or citizens (and there is a distinction between the two categories under Mexican law).