You've got a tough situation there. The good news is that your M3s will sound much better than what you currently have, even if you have to mount them in the exact same location as your current speakers. Is there any possible way that you can rearrange the room? I realize that you have a corner unit for the TV, so that's probably out of the question.
Given what you've got to work with, you can probably get a pretty good stereo image from some ceiling mounted M3s as long as you point them down toward your listening area. You might be able to use Axiom's Full Metal Brackets to wall mount the M3s, but I'm not sure if they would clear the valance for the vertical blinds.
2x6 commented about vibrations coming form the piano strings being an issue. That's likely to be true if you place an M3 on the piano or if you like your music loud. At everyday "wife-friendly" volumes, it shouldn't be an issue.
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