Would who or whom be used in this phrase?

Whom

A general rule of thumb is to turn the sentence into a question. If you would answer the question with HE, then use who, if the question would be answered with HIM the use WHOM. (Remember both HIM and WHOM end in -M)

The driver who/m drove the car was drunk.

Who drove the car? He did, so The driver who drove the car was drunk.

I just fired the plumber who/m you hired yesterday.

Who did I just fire? HIM So I just fired the plumber whom you hired yesterday.

Who will we call Melanie for the sake of the story? HIM. So The child whom, for the sake of the story, we will call Melanie....

hard to give you guidelines as who is gradually replacing whom in everyday language and whom sometimes seems outdated. personally, i'd use whom in this context.

"For the sake of the story we will call HER [not SHE] Melanie." "Whom" here is the object of the verb "to call." So it's whom, not who.

Who is correct.