1 is ok, but it is based on a strong body. For example, the limit method (linear problem) has friction in the square and no friction in the vertical, so the larger the angle, the smaller the friction.

This is a law, the specific principle requires knowledge of calculus, so high school only needs to grasp the conclusion, of course, it can also be understood by limit method, that is, uniform motion is this law.

This is Newton's law, a summary of the law, or a definition of force, which makes no sense.

This is also the conclusion, especially the linear uniform acceleration motion. Refer to the second question for specific principles.

To sum up, you think too much, get to the bottom of it. There are many basic theorems in physics that are the summary of people's long-term experience. There is no reason or specific principle that requires very complicated mathematical tools, so be careful when reading books.