Hi, mtpaper.
To answer your questions:
1, 4. Yes, the 3G is only limited to the Sony Store. You need Wi-fi to access any other websites like gmail. If you were traveling on the road, then you would have to stop somewhere with Wi-fi like McDonald's or Starbucks.
2. The Sony web-browser comes with a preset link to Facebook so I assume that means it works. I don't use Facebook so I can't comment how well it works. Again, it would only work via Wi-fi and not 3G.
3. Library books have to be downloaded from your library website on your computer and then transferred via USB cable to your ereader. You cannot wirelessly download library books either with Wi-fi or 3G. You can get newspaper subscriptions from the Sony Store via 3G wireless downloads on the 950. You can find there selection here.
http://ebookstore.sony.com/category/newspapers
Hope that helps!