A. IE 8's Compatibility mode emulates IE 7, not IE 6. When you enable compatibility mode (by clicking the broken page icon next to the website URL), IE tells the site you're using IE 7 and emulates IE 7 behavior. You can see this if you look at the appVersion information that's sent from your browser when in compatibility mode, as shown here.
If you need IE6 on a Windows 7 desktop, the best option is to use XP Mode and run IE 6 in a virtualized Windows XP environment. In an enterprise environment, you can use Microsoft Enterprise Desktop Virtualization to maintain and manage the XP image.