I undervolted to -160mV. Installed SolidWorks CAD software and the noisy fan kicked in even the system profile stated that the CPU load was 1%. The X1 Yoga Gen 3 and Yoga C930 did not have such loud noise when running the same program. rMBP 2014 15" also did not have any noise issue. I could try to do a thermal repaste but the result may be the same. I guess I may have to give up on the X1E :-(
When you say the fan kicked in, what do you mean exactly - was it running all the time you were using the programme, or just some of the time, or just occasionally?
Regarding your questions about the undervolt not 'sticking' there are loads of guides out there about how to set a process for Throttlestop so it automatically runs on startup.
Out of interest what was the temperature saving from your undervolt (I presume you tested both before and after).