Hello Cosmos,
Thank you for your message.
1. Both Windows and Linux/Mac Viewer use the same address book format. You can use the 'address book manager' (available in the address book tree context menu) on Linux/Mac the same way you can do in the Windows Viewer.
2. This might not be immediately available because currently the Linux/Mac Viewer has more limited functionality than the one for Windows.
3. Yes, it is. Same license key and same registration process.
P.S. I'm saying 'Linux/Mac' because the version for these two OSes shares the same codebase. In most cases when we update a version for Linux that means that an update for Mac is also available, and vice versa.
Thanks.