Sorry, JB, the Homasote works. About a year ago, we put a layer of Homasote under the stairs on my kids house with a layer of plywood over (or really under) that. They had been having trouble sleeping because of the noise from the washer and dryer in the basement. With the Homasote, no more problem sleeping.
I did not say to put the Homasote either on the outside of the house or on an inside wall. I said to remove the drywall, replace it with the layer of Homasote and then put drywall back over that.
Your method will work but I doubt that it would be allowed for most condos or apartments. I also agree with your comments about the windows being a major sound transmitter. But, again, will Brebda be allowed to install different windows in his condo?
The Homasote under the drywall will deaden some of the sound and will subtract only 1/2" from the size of one room. You can work around this for the window trim so you will barely know that it's there.