No, mechanical keyboards are merely “nice to haves” and not mandatory in PC gaming. The money you set aside for a mechanical keyboard should go to an SSD or a better GPU instead.
No, all scientific studies correlating extended screentime with any visual impairment are yet to establish a causal link. It is premature to arrive at the conclusion that screen time leads to myopia.
Yes, if you are asking this question, it means that you have recognized a problem in your lifestyle. Playing video games is not evil by itself. Having it control your life to the point that you are questioning yourself is a sign of waning self-discipline. Some other hobbies and activities are far better instead of playing video games.