In regard to your first inquiry, I just wanted to let you know that Intel tested the NUC under stock configurations with default settings, based on that there is no documentation explaining how the NUC will work if there are changes on the voltage parameters. To answer your second question, we actually recommend to use the NUC that same way and to avoid any voltage changes on it for the NUC to work properly.
Altering clock frequency or voltage may damage or reduce the useful life of the processor and other system components, and may reduce system stability and performance.
However, on the following link you will find information related to the voltage range for the processor i7-6770HQ, page 110:
I also encourage all the peers viewing this thread that have any information related to undervolting the NUC, to post any comments or suggestions related to this matter.
Any further questions, please let me know.