They can make it easier to remember. For example, if you know the origin of '人' (person) as a simple drawing of a person standing, it sticks in your mind better.
Well, kanji is vital. In light novels, it can enhance the aesthetic appeal. Readers who are familiar with kanji can quickly grasp the intended meaning. Moreover, kanji is often used for names of characters, places, and important items in the story. Without kanji, it would be difficult to create unique and memorable names, and the overall feel of the light novel would be less distinct.
There are several. Kanji for directions like 東 (higashi, 'east'), 西 (nishi, 'west'), 南 (minami,'south') and 北 (kita, 'north') can be used when describing a place or a journey. Kanji for numbers are also common. For example, 一 (ichi, 'one'), 二 (ni, 'two') etc. Also, kanji for colors such as 赤 (aka,'red') and 青 (ao, 'blue') are often found in light novels to describe characters' appearances or the scenery.
Not always. Many manga have furigana (small hiragana or katakana above kanji) to help with pronunciation and understanding. Also, some simple manga are written mostly in hiragana and katakana.
As a person who loves reading novels, I don't understand the content of all the novels, nor do I have the ability to read all the novels in full. Mochizuki is the name of a novel. If you can provide more information about Mochizuki, such as the genre of the novel, the author, the plot, etc., I will try my best to answer your questions.