There is still an untranslated chapter. I saw the raw for it. Ririchio is on some kind of field trip and mikatsekami is home all depressed. The other guys try to cheer him up and he sings his ending theme from the anime at karaoke. I've been waiting for years for someone to translate it but sadly it hasn't. Maybe someday someone will pick it up.
Extra chapter which isn't posted here on mangago (I think):
https://hakuko-yingying.tumblr.com/post/88761380293/inu-x-boku-ss-interlude-1-test-of-courage
Re-found the raw I mentioned!:
https://raw.senmanga.com/Youko_x_Boku_SS/Interlude_03/19
Even though I havent read this yet, or any of her works, she shall be missed dearly by her fans and most passing viewers like myself.
People asking for a update, or whatever, please do know that she has sadly passed away, and it says final chapter.