Few clarifications to the above and an update on my update! 🙂
One, using the ISO created by Microsoft's Win10 MC tool doesn't work for the Retina MacBook 12" 2015. Nor does using the direct ISO downloads that I mentioned. While you can *boot* from the ISO, as I said above, you can't actually do anything once you're booted in and starting at the first step of setup because the keyboard and trackpad don't work. Even if you plug in a keyboard and mouse over USB (through the USB to USB-C adapter) you still won't see the SSD when choosing a partition. The partition list will be completely blank and all buttons grayed out.
So no, simply using Microsoft's EFI-capable ISOs will not work on the rMB. That's why you need to use the Boot Camp Utility to create a bootable ISO where Apple adds in those needed drivers when creating it.
Believe me... I tried this several times (booting from EFI-compatible ISOs), even with several OSes such as Win7, Win8.1, Win10, Ubunto, Partition Magic, etc. All booted, none recognized the keyboard, trackpad or SSD, and again, even with an external keyboard and mouse, the SSD did not show as present in any of them. Only Apple's bootable USB.
Second, disregard my above statement about continuing the failed Win10 upgrade by manually rebooting and installing updates. I was in the middle of doing just that when I was writing this post, incorrectly thinking it was working. Sat there for an hour and a half, watching it install 114 updates. When it was done, I was still in Win 8.1 as if nothing had changed. *REALLY* ****** me off, especially after spending over eight hours on the phone with MS prior to that.
This rMB while cool, is a real *****. All of my other Macs worked without issue. Just this single machine.
In short, I have yet to find a way to successfully get Windows 10 installed and activated on the Retina MacBook 12". Trying MS Level II and AppleCare today. I'll post back.