I was definitely wondering about that second boss in the game screenshots. I had figured that the boss at the end of the week would be random and didn't try playing the game again to find out. I might give it another go at some point and see if that hidden boss will show up for me. Regardless, I do personally feel that the bonuses granted to the player by the random items is really strong in relation to the nights that just feature regular enemies. Given how dangerous the main boss became towards the end of the fight, I could see the enemies being adjusted to be somewhat more dangerous as well.
This is going a little off-topic, but I did check out your team's profile page. I really like being able to read about the people who develop games like this one; it's great that the two of you are so passionate about what you do!