I found this cool artbook related to the series:
https://e-hentai.org/g/961331/1f300a59f9/

I'm pretty sure it's official, the art is the same, and the artists listed are the same as the ones responsible for the official series.

The character designs were made by Range Murata and Takuhito Kusanagi. Looking at their other works, I would imagine Range was responsible for the general style of the series and Takuhito for the more 'monstrous' elements. Though again, it would be cool to dig up more information.