Yes, you can hook up a 2.1 solution.
Yes, you can use a USB audio solution. I have used a couple of models with the NUCs: Amazon.com: Sabrent USB External Stereo Sound Adapter and (slightly better) Amazon.com: Sabrent USB External Sound Box. I have had absolutely no issues using either adapter and, during my testing, have used them with every NUC model. The sound quality is not bad for something so cheap.
Yes, you can use the audio feed from the NUC as well (with the right cable).
Hope this helps,