Enforce streaming service encoder settings makes sure that you adhere to the streaming sites video requirements, here are the ones for Twitch but the others like YouTube and Hitbox have similar types of requirement.
http://help.twitch.tv/customer/portal/articles/1253460-broadcast-requirements
Always leave this enabled is my advice wherever you are streaming to.
x264 is always best (IMHO) but needs more CPU, if you don't have a lot of spare CPU then try NVENC, no one can tell you which is best as the actual answer is "It depends". Experiment with both in your setup and see which one gets you the best results.
I don't know the answer to your last question but I would recommend not messing about with it unless you know what you are doing. I know for streaming this must be off, for recording take a look at this article for high quality:
https://obsproject.com/forum/resour...lity-recording-and-multiple-audio-tracks.221/