To answer your First Question, the game's servers have been killed off. No-one knows what has happened to them, but we think that Microsoft/343 have intentionally killed them off (They haven't been responding to messages sent by many of our members)
To answer your Second Question, I believe that there is no possible way to 'Backup the activated state' if you reinstall the Operating System (Via Clean Install). If you installed the Operating System in a way where you can Update (Like Windows 7 - 10, where you can keep your files and programs installed and activated) Then that should work for Halo 2. Besides, when you Un-install Halo 2, you get one more activation, which means you could reuse that activation again (I can vouch for this, I installed Halo 2 and Maxed out my limit of Activations, then un installed and Deactivated Halo 2 on those PC's and I was allowed to use those activations again)