Responses

I think the main difference is that both the novel and the anime intercalate between the point of view of Kumoko (our MC) and Shun, and I think this is important. It is part of the puzzle that the author created and, for me, it would be the only flaw in the manga. Other than that, the rest are small details that don't change much, an example is in the novel Kumoko saying "I fought with the children that the mother sent after me" and that's all while in the manga there are several chapters of this fight
What's wrong if the anime adapted the light novel not the manga the novel is the orginal source
I think it show's that they care about the original Novel stans and the orginal work n I guess most westerns only know the manga but I think most hardcore stans will be happy that they decided to adapt the novel