I thought you meant flat wing as in no dihedral.
You must be talking flat as in the QHor, Gnat or Spa3D. I've never seen anyone make a BUHOR with a flat wing and no airfoil.
As for engine angle, I've never seen enough of a problem to worry about the offset, but I'm not the most graceful flyer out there.
Most everyone on spadworld.net just recommends to mount the engines straight so that's what I started doing and it's worked out ok for me.