Yes, you can get to 100 percent mastery in a course without doing the course challenge. As you suggest, it depends on if you mastered the skills or not. To achieve mastery in those skills, though, you will need to successfully complete them in either the course challenge, unit tests, or mastery challenges. You can’t just level the skills up to mastered by doing their individual practices.

Yes.  I have done this numerous times. In order to get 100% in a course, you have to master every unit completely. First finish each question and then do the unit test.