Most of the time, it's not a rule that every manga has a light novel. It depends on various factors like the popularity of the story, the author's plans, and the publisher's decisions. Some manga are so unique that they don't need a light novel version to tell the story effectively.
Definitely not. The existence of a light novel for a manga is not a given. It's not a one-size-fits-all situation. Some manga concepts might not translate well into a light novel format or there might not be enough demand for it in the market.