強い(こわい)usually does not exist in Japanese.
It is almost always read as 強い(つよい)(strong).
If we hear こわい, it would mean 怖い (scarely) in most of the cases.
強 read as こわ is an exception.
It is only used in a combined word such as “強張る” and "強面(こわおもて)".
強(こわ)means "tense," "nervous," "strained" in your context and "scarely" in 強面.
On the other hand, 硬い means "hard."
The meanings might be overlapped, yet I don't think they are the same.
硬い顔 and 強張った顔 mean more or less the same, while 強面 means different.
BTW, こわいvsかたいについての問題 質問です
問題 is for questions in an exam or quiz.
質問 is the right word choice here, although both are translated as "question" in English.