DVSVojvoda,
I haven't seen the drawing (fireplace/chimney architectural detail) either. But I just imagined a standard fireplace with say masonry chimney that stays in touch with the exterior wall all the way to the roof. I believe that the insulation between the fireplace and the chimney (in the space above the mantle to the story-high wall)should protect a drag strut. But regarding consideration of one shear wall versus two, I think there should be some continuity in the shear transfer through the diaphragm and the walls.
There could be a new way of doing this that I am not aware of. In the past what we have always done is have a opening (just like a door) and the fire box will get framed in the opening. As the fire box rises to the damper and smoke shelf it gets built out ward. Once the box is out far enough or to specification it should clear the out side wall and top plate. The flutes get added to height that is required. The hearth and mantle are built on the inside. This will leave the top plate in place to act as a strut. You can then add the front of the fireplace. If the face of the fireplace get brick or stone all the way to the ceiling just increase the height of the header (Of course you will need a header just like a door or window opening) and top plate will get sandwiched in the brick. All the wood framing gets insulated by the brick and flutes. It also seams that as the firebox get built upward and back you will need to add metal lentils to support the brick. I have a detail of this but no way to post it. It you have or know of a way to post a "pdf" file I will gladly post it or send it to you.
