I agree that the knowledge that multiple Seugathi was a TPK in the making and yet was not addressed by the writers was a big mistake.
I think that the difficulty of getting into the house does encourage trying to do it in one go, and that the final house encounter is another TPK in the making. I'm not sure how to balance that other than by making the house easier to get into.
I didn't quite get the demi-realm and the resolution was very unclear to me (likely because it's been so long between this episode and the prior ones). The so-called deal between the brain thingies and the Pathfinder Society(???) was also highly confusing.
Why was the XP divided by three, not four? I don't remember if/why Alex is getting treated differently than Brunsk?
Regarding the house, it's not difficult to get into at all. The stairs are totally unguarded. To get into undetected however... But I did try to stress that he's a rogue element. He's not getting any more help other than what he can scrounge together. His own family was totally ok with him getting murdered to end the feud, provided their property in his possession wasn't touched. The first floor of the house is also very static, very few things leave their room to investigate combat, lending itself to a stop and breath situation. Constructs, once broken, remain so for the most part. Admittedly an assault that leaves all three living characters alive could be dicey to return to, but even that would most likely be an assault by the harpy and cat girl while Berkanin remains in his study until interrupted. No reason a retreat was not feasible. The TPK was mostly unluck, and one part the difficulty of a three person party, rather than being too difficult.
The Demi-plane is just that, it's own little plane, and one that was structurally unsound thousands of years ago. It functions by its own rules and is not part of the material plane. In many ways it's kinda a mini Ravenloft. Karzoug agreed to shore it up with the shard to have the Dark Rider also bound to protect it from attempts to steal it. He also put the bonesage simulacrum there as part of a study agreement I believe. Likely more expansion was planed (Karzog was quite the one for plane exploration and making these little Demi-planes it seems), but then the whole earth fall and stuff.
The deal between the Caulborn and the Librarians at the spire (can't recall proper name. Not pathfinders) is kinda meant to be vague, the PCs weren't there at its initial agreement, and it's possible either the librarians duped the party, or the Caulborn duped the librarians. Either way, the Caulborn wanted into the Dark Forest but Karzog's prop with the shard prevented their entry, but not the entry of humanoids, hence why they reached out to the librarians to get the shard removed. Whether their claim of wanting memories of the group's time in the dark forest is true, or merely a rouse to gain the shard for themselves, is a mystery or up to interpretation of the PCs.
The exp has always been split three ways. Alex is a cohort, like Brunsk turned into.