I'm not sure where the exact cutoff point is, but yes, the chapter 1 monsterpedia ends after Gnome. It doesn't matter which evaluations you see or how you see them as long as they're all from chapter 1. The 50th evaluation gets replaced by the scene so you can't unlock it by losing to Imp 50 times in a row, unfortunately.
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On a completely unrelated note, I probably know more about the small details of Imp's fight than anyone else now.
I double checked and it seems like there are actually 2 episodes. They were also better than I remember, I could have sworn there were only succubi and Erubetie in the OVA but the second one has more variety so I probably never watched it. I'd probably say they're pretty decent now, I'm pretty sure Luka even gets to use the japanese version of Ninefold Rakshasa! The version I watched absolutely butchered some of the names in the english subtitles though, I don't remember anyone being named Elvetti at least.