3.5" round box for wall sconces, cannot make work with many fixtures
I believe that an "offset mounting plate" might work for you (click for larger):



Sample swivel offset crossbar

In this type of plate, the front and rear elements are offset from one another so you can have the box mounting screws and the studs for the fixture practically on top of each other.

The box does look slightly recessed into the wall which could work in your favor here.  The only issue would be the sconce sticking out too far from the wall, but you may be able to flatten the bracket some to account for that.

There are other styles of this bracket that could work better - I just grabbed the first example I saw.