So near as I can tell the bonus page is the tsuchinoko surrounded by natto rolls (food). This is only ever so slightly relevant to next chapter, so I'm not sure why it was placed here in the volume.
Yeah this series is definitely gunna be good comical ride. The only thing I didn't like so far was 3 with the Mom thing but it's not that bad and still had a damn funny opening joke. Chapter 2 was the best though and none can change my mind.
Her thought about "running away from reality" seems like it might hint at more serious topics in the future. Like why is the MC so into Uma's and the occult or just so believing of that?
I was expecting the title to be a play on words since hentai also means metamorphosis, but the title actually uses metamorphosis written in katakana (メタモルフォーゼ).
Thanks for picking this series up! It's been fun and cute!