1. Both Windows and nVidia expose 1024x768, 1152x864, 1280x768/800/960/1024, 1360x768, 1366x768, 1600x900/1024/1200, 1680x1050, 1920x1080/1200/1440, 2560x1440
2. In addition to those, games expose 800x600. I don't think I'll see the need to use a lower resolution in games, because G-Sync does seem to alleviate scenes with naturally lower framerates.
3. My SD videos (640x480 ish) look worse than on my 1920x1200 Dell U2410; you can notice more artifacts. My 1920x1080 videos look the same as my other monitor, and better than 1280x720 videos. I'm not sure if I understood this question right.
As for G-Sync with video players, most won't use it by default because G-Sync only works in exclusive fullscreen mode, while most players actually use a borderless windowed mode in "fullscreen". However, I think this is preferable, since when I tried enabling exclusive fullscreen mode in Pot Player, it seemed to make the quality worse. I believe G-Sync doubles some frames to meet the minimum refresh rate for the monitor, which is higher than 23.976, which is not ideal. The videos stuttered.
In my profiles, I disabled G-Sync for my video player and browser (because of streaming videos). It really does suit games though. If your videos are higher than 30fps, then G-Sync would be better.