Subject-verb agreement !? ><

I have trouble with the verb that comes after " the only one of " and "one of" if it is singular or plural.

Here's an example,
Carlos is the only one of those students who ________ lived up to the potential described in the yearbook.

The answer is "has".. and in the website , where I got this example , explained it as ' The 'who' refers to 'the only one,' which is singular.'

But, I know that we decide from the word that comes after 'of ' which is plural.

Is it only because there's 'the only one of' in the sentence?

What if the sentence was ..
Carlos is one of those students who ________ lived up ...etc.

Will it be "has" or "have"?.. please explain!!
To me, 'have" is correct because it refers to "those students".

One more thing, Is there any change in the verb (singular/plural) if the word after "of " was "of those students" or " of the students" in the example above? or it will remain the same?

Thanks. :')