I believe the entire multihead section is far more complex than necessary for a 9350 model XPS 15 w/ hidpi display. HDMI, DP->DVI, and the internal monitor all "just work" at the same time with a mundane arandr configuration (nvidia card off, intel only)
Since I've not tested plain display port, nor DP->VGA yet, want to see if anyone has wildly different experiences with that model before I do any edits?
I agree with Thomasam. I have the newer 9550 model (Skylake) and both, the HDMI as well as using a USB 3.1 Type C/Tunderbolt 3 to HDMI adapter just work (all at the same time and perfectly fine if plugged/unplugged at runtime). I even don't have any X11 configs. Just make sure you apply all the recent BIOS and Thunderbolt firmware updates and use a HDMI cable that is known to work (I had a rather frustrating few days with a brand new but broken cable which sent me down many misleading rabbit holes including this wiki page).
Issues with sound (9550 Skylake ALC3266)
I wrote the section regarding sound. I will attempt to clean it up and conform it to the guidelines.