1) Does the game run normally when installed via GoG? I ask because as everyone here is acutely aware, this game was notorious for its buggyness.

2) What exactly does nglide do? Is it better graphically to run the game using nglide over OpenGL solely for higher resolution support?

3) Was SiN ever open sourced? The Quake 2 engine, upon which SiN runs, was open sourced over 15 years ago.

4) Are there any other obscure settings that need to be tweaked to get SiN working correctly?

It does. The bugs were largely fixed with the official post-release patches, you know. http://www.zeus-software.com/downloads/nglide Parts of the official source code were released, but nobody made any source ports that I know of. Not really.

You may want to check the SiN Ultimate Patch thread. There it is explained how to get a bigger HUD etc. when playing on higher resolutions (e.g. 1920x1080).