No, not an error.
Ill look into the hangers, see if i can make it work.
I have changed the design a little which might help too.
The shelf will be in each alcove either side of the fireplace and will now run straight across the chimney breast creating a mantle above the fire.
Make sense?
This means i can run one long timber from wall to wall, bolting to the face of the chimney breast.
I could even be tempted to cut a hole in the wall to support the ends of the front beam.
Once all framed up and fixed the whole lot will be boarded then veneered in something nice.
I may very well be over thinking this, but better to be too strong than too weak.