Just want to say you did an amazing job with this update
@Tabula Rasa especially since it has lead to such an interesting discussion.
Alright, so when it comes down to it we are gonna need a guide if we choose to explore this place again. Something I see as being very likely to happen considering how it relates to the founders and is just super interesting. While the Imp can not enter Hogwarts, nothing says it cant guide us through the dugeon, something we will absolutely need if the challenges from before are anything to go by.
Should we decide to make a Pact I think that Jacob will go about making a very ironclad set of terms and conditions, ones that should prevent the worst from coming to pass. Killing it may be tempting now, but that is shortsighted in regards to figuring out what exactly is going on here. Once a pact has been made, and it becomes saner we can ask it a few more questions regarding this location, as well as what dangers we may face in the future. That way we know what to prepare for and start getting mastery over some more spells, just in case. (Deep Understanding Protego, Patronus, etc...) Perhaps even some alchemy items as well might help, such as seeing any hidden dangers and a bonus to combating creatures. (Ghostly Goggles)
As for the Dark Arts I like to think that they have a fraying effect on one's sanity and that we should touch the skill sparingly. However, if we want to explore this place further and make the most of it a higher skill in the subject couldn't hurt. Overall my point is this, we need to make a pact with the creature since its knowledge and assistance since its help will give us a better chance of making it farther into the dungeon and learning just what exactly this place is.