There has been no evidence that George Romney ever took the proper follow up actions. That was part of the problem back in '64 when he was a POTUS candidate. There were rumblings about his eligibility back then but he quicky dropped out of the race so the questions were also dropped.
Mitt has been shoved down our throats for a reason. That reason is because the GOP is cya-ing over allowing the Obama presidency stand. Once there's a GOP ineligible president then "they" have won. That old outdated Constitution will finally be shredded and it's welcome to the New Amerika.
IIRC, under the 1907 statute in force at the time George Romney repatriated at age 7, he did not need to reaffirm his US citizenship or get naturalized to remain a citizen. If he had not returned to the US until after turning age 18, I believe that he would have had to do so.
The questions about George Romney were about whether a person such as McCain who was born off of US soil jurisdiction could be NBC, not whether either of them were citizens.
If the wildly expansionist Marguet-Pillado standard were to be applied the both George Romney and John McCain would be NBC. (not!)