Very very close characters. 那 is that, but 哪 with the mouth radical in front denotes it's a question 哪个（which) or where （哪儿）in the same way other question characters have this mouth radical out front
How could one spot the difference in a spoken sentence? Seems like it could be hard to differentiate.
Especially as the 3rd tone chabges to the second in some cases in which case it'd be impossible to tell the difference
Could this also be intererpted as plural, i.e. "Which male celebrities do you like?" or is it unambiguously singular? I typed it in in plural and it was marked wrong, but I wasn't confident enough to report it.
Your translator is inflexible and, I suspect, does not live in the real world. 男明星 is a male actor but not every 男明星is a celebrity or star. I wrote “Which male actor do you like?" and I was marked wrong.
明星 only refers to people in who are famous in show biz, for the literal meaning "bright star." Also, 明星 isn't necessarily actors. Singers, models, or even sport players can also be 明星 if they have many fans.