Forget about diffusion there. In fact, in this small of space, diffusion is all but impossible due to the dimensions of the room. You need all the absorption you can get in that small of space. In fact, you need THICK absorption as well. That room is a tough one, as your early reflection points are very close in that "closet" area and on those side areas, you'll get a "coupled space" effect, which amounts to a "delayed" reflection from the wall opposing walls, due to diffraction around the corner. I would put up a few 2" thick absorbers on them as well as these reflections will be out of phase and "re-diffract" back to the listening position. I would also suggest a ceiling cloud as well. Six feet is very very narrow and I highly doubt if you can get your monitoring any where near "truthful", especially because of the dual coupled spaces.. Best you can do is make it dead around the listening position and hopefully learn to identify the limitations of your monitors in that space by judicious amounts of listening to your recordings POST MIX, on other systems and rooms to see how well they "translate" and learn from them what is working and what is not.
Also, you might want to use a series of 4" thick 1 or 2ft square absorbers in a checkerboard fashion on the rear wall. This will give you a modicum of "diffusion" there, but mostly to break up the specular reflection from this wall.
I'm sorry, but there are no perfect answers to rooms such as yours unless you had a professional design it and even they might pass due to the problematic room layout. Just use what you have and do the best you can with it. Don't be afraid to move treatments around either. Remember, the whole point is TRANSLATION. Learn what your monitors are telling and NOT telling you is the important thing. Your room should not influence what you are hearing, but I believe this room will be very difficult to treat enough so it doesn't "color" what you hear in the monitors. In this case, absorption is your friend. LOTS of it.
Well, that's my best NON EXPERT .02
fitZ