Can't imagine how a single pair could diffuse. This style of diffuser would require a bunch of them at a great distance away. Note that diffusers must be very rigid and non-absorptive to function.
Yes, the speakers should be at least 3 - 5 feet from the front wall (behind them), as should the listening position.
A non-symmetrical layout could work, but not in your room with your speakers (much too small with dipole speakers, TV, and a couch). The sound and picture must coincide to avoid a sensory disconnect.
You have my suggestions, first measure to better define the problem (beyond sounding "echo-y"), before wasting money/effort fixing an unknown problem.