No, the story of Kannagi is likely fictional. It's usually created for entertainment or to convey certain themes and ideas rather than being based on real events.
The decline in popularity of Kannagi manga might be because the characters weren't developed enough to keep readers emotionally invested. Another factor could be poor marketing and promotion, resulting in less visibility in the market.
Well, without having read 'ayakashi oyado ni yomeiri shimasu light novel', it's hard to give a detailed account. However, light novels often have unique storylines. The 'ayakashi' part makes me think it could be a supernatural story. Maybe the main character goes to an inn ('oyado') and gets involved in some sort of strange events related to ayakashi, and the 'yomeiri shimasu' might be a key part of the plot, perhaps symbolizing a new start or a special bond within the story.
I'm not privy to the main characters in 'ayakashi oyado ni yomeiri shimasu light novel' since I haven't read it. However, based on the title, it could be a young person who, for some reason, has to stay at the 'oyado' where the ayakashi are. Maybe there's a female lead who is trying to understand and deal with the ayakashi she meets there, and her experiences could form the crux of the story. Or it could be a male lead with some special abilities that make his encounters with the ayakashi and his stay at the inn very interesting.
The main differences are in the pacing and some minor character developments. The anime might speed up or slow down certain storylines compared to the manga.
The charm of Planus Girl manga lies in its unique world-building and the depth of emotions it evokes. The illustrations are beautiful and the themes explored are both relatable and thought-provoking. Also, the way the story progresses keeps you constantly curious and invested.
Love report manga often focuses on emotional depth and character development. Compared to some other romance manga, it might have a more unique plot or art style.