Copyright and Fair Use

Hello, there!

I'm a teacher and I'd like to put up an online course to sell to students.
The focus would be English for beginners. Part of the course's goals is for the students to understand English movies without the subtitle of their own language.  
I'd like to add 10-15 seconds of Youtube movie clips for students to understand English native speakers in different situations. My problem lies with the copyright and fair use. For a long time, I've researched on this, but I couldn't find a concrete answer. Upon understanding it,  Fair Use says that I could use videos for educational purposes and if it's beneficial to the owner. A downside is that I'm going to use it for commercial purposes.  
I asked permission from these Youtube channel owners ( such as Fandango ) but they haven't answered for months.
The platform wherein I would put my course says that they don't give advice on this one and they won't remove what I put on my course.
I also asked a lawyer from the USA and he said that I should ask advice from my country and this is why I post a topic here.

Thank you for reading this far.
Have a great day!

Economically benefitting from the work of another even with the proper citation might be considered a violation of fair use.

Question:
Example, there is a textbook protected by a copyright, I answer all the questions in the exercises at the end of each chapter. Then I compile all my answer into a book highlighting that this is the answer to all questions in that textbook, then I publish it.
Am I violating a copyright?

Thank you very much!