Yes, that one coin difference is needed to make the further decision of whether the bad coin is light or heavy.
Thank you, and thank you for the puzzle. It was fun to understand the whole area and clarify my thoughts. For one thing, I learned about how general the restricted case is. While we all know that, if the bad coin is known to be light, we can find it in n weighings from 3^n coins, I hadn't realized the more general result (which I now proved) that it doesn't matter whether the bad coin is light or heavy, as long as we know which ones could only be light, and which ones could only be heavy.