Otherwise, by making the case from the outside, it almost works, I get this:
But it's not enough, I don't want this kind of ring (the basis of the little revolution)
By the way, what shape do you want to have on top? because you want to cut the red part, but how far and how?
And for the guide curves, do you have the right to make them tangent to the upper sketch, if you manage to find a solution to the hull.
2015-06-16_export_modifie.jpg
I only wanted to cut out the flat part of the top, which I can do by doing an extruded material removal on the oval sketch (which is on plane 13 if I remember correctly).
Indeed it works in your room, but I can't see what you have modified, wouldn't there be a way to superimpose your room and mine to compare? :D