LightNovel.org

Also there are 3 illustrators mentioned on the website (with images of their illustrations, which look pretty great :) ), though at this current moment its not mentioned which light novels they'll illustrate.

It is illustrated after all, so I consider it to be a light novel (as also other japanese fans do consider it so). Though, it is not published under a light novel label (like most light novels are) but it was published under the "Kadokawa Bunko" label, a label for publishing novels.

But really I wouldn't like to enter this type of discussion with anyone. stuff like: "it doesn't have enough illustrations to be considered a light novel" or "the writing style is different from usual light novels", stuff like this is the type of stuff I keep away from...

Kodansha Novels (for example) is a line for publishing novels, but has published lots of novels that are considered to be light novels by fans.

I, like other fans (japanese fans) will call whatever looks to me to be a "light novel" a light novel, even if there's not enough illustrations and even if its not published under a light novel label, and even if the writing style looks different from other light novels' writing style...

Anyway, if you want to go with what the publisher says (which many prefer to do this) then NHK ni Youkoso is a novel, but if you want to go with what some japanese light novel fans would say, then its a light novel.