my best guess is that choosing a door only increases the difficulty of the rooms, but they stay the same?
that would explain why his brother knew about all that, but as to why the dagger was still there even tho he knew about its abilities, its either a he didnt want it because he saw it as a evil weapon, or like i mentioned before, he learned about it later on, and regretted not taking the dagger, since i think you could only get it in the tutorial stages.
allot of speculation on my part since the author didnt explain jack s#it, but thats the best i can come up with.
~Never die Alone~