Just make the adnsese background color the same as your page background. edit-I have not read your first post that you have a multiple color background. But I think you just have to compromise with your design to accommodate the ads so that it will blend properly with your color theme.
I don't believe it is possible, I looked into this as well, Google TOS clearly states the background must remain proprietary mfusionxl
I think there is no transparent background for adsense. Just try to make it close to your page background
I think this is a security method. Imagine what if they allow transparent background and some people start adding the code on CSS'ed images and so on...