In Konosuba, the OVAs are indeed cannon to the light novel. The OVAs are created to supplement the light novel's content. They often contain elements that are in line with the original story's tone, themes, and character development. The creators of the OVAs take care to ensure that they don't deviate from what has been established in the light novel. This way, fans of the light novel can enjoy the additional content in the OVAs as an extension of the story they already love. Also, it can attract new fans to the series as a whole, both the light novel and the OVAs.
The Konosuba light novel is not a fanfic. It has its own unique plot, characters, and world-building developed by the author's imagination and creativity.
The Konosuba light novel is good. It has a charming mix of fantasy elements and comedic situations that make it highly enjoyable. The character development is also well done.