Hello Ed,
I just responded to your email, but for posterity's sake I'll also reply here.
I think putting the HF MiniTraps on the back wall, and the diffusors at the back 1/3 of the side walls make the most sense given your situation. Having the additional bass trapping on the rear wall makes sense, there is usually a bass buildup there. And the diffusion won't hurt the piano sound at all.
The other possibility is to put the diffusors on stands, so that they can be centered on the rear wall. If you go this route, you'll want to put the HF MiniTraps on the rear wall, to the left and right of center (where the diffusors on stands go). Perhaps you could mount one of the HF MiniTraps to one of the doors?