Why could Nicodemus' confusion only occur if Jesus used a word that could mean either "spiritual birth" or "physical birth"?
Assume that Jesus used a word that meant "born from above."
You said, "If that’s what it means then Jesus is telling Nicodemus that he must have a birth from God above if he is to see the kingdom of heaven."
Why can't it just be that Nicodemus didn't understand what Jesus meant by that?
Even today, using English, the Watchtower's understanding of what Jesus meant by this "birth from God above" (designation -- whenever it happens -- to be one of 144,000 who will go to heaven instead of living on a paradise earth) is very different than what evangelical Christians understand it to mean (an inner transformation by the Holy Spirit which occurs at the time any person surrenders his or her life to Christ).
With respect, even your own explanation of the term is ambiguous. What is this "birth from God above" that some people have and some people don't? Is that something that takes place -- if at all -- at the time of physical birth? During a person's childhood? Adult years? At the moment of conversion to Christianity? At the moment of death? If it's something you're either born with or not born with, then Nicodemus was out of luck. What was he supposed to do? Go back and start over? Isn't that a reasonable explanation for his question to Jesus that has nothing to do with the differences between Aramaic and Greek?