Hi Headhunter06,
I experienced the exact same problem described by you and so many others here. I followed the steps of installing W10 through BCA without connecting to the Internet and turning off Auto update all together. After a couple of trials I managed to get a stable installation, so first of all thanks very much to you and others in this discussion for getting me this far.
THe unfortunate thing with this solution is that now you won't have any updates at all, which is not entirely satisfying to me. Searching the internet I came across a howto that tells you how to exclude a device from having driver updates on the basis of its hardware-id. It's done through the local group policy editor. Here's the link: http://www.makeuseof.com/tag/take-back-control-driver-updates-windows-10/
I did this and it seemed to work. To be sure of that I restored the first stable with the Winclone image I made of it and repeated the procedure and hey presto! It worked again. After turning back on the auto update the first updates started to download and install but for the Iris 6200 update it gave the glorious message: installation failed. Exactly what you want to see. I had to share this with you all and I hope it helps you too, at least untill MS closes this loophole. From what I read I get the eery feeling that they are on a zealous mission to take total control over our machines.
During this process I also found out that the iMac still works ok when you use the Microsoft basic video driver instead of the Intel driver. So if all else fails you could decide to go with that. I didn't test it any further but when after the first connectionless installation you go to the device manager, look foor the graphics adapter and choose the option to roll back to the previous drivers, it gives you this basic driver, the one Windows installs before you install the Apple WindowsSupport files. I guess you won't be bothered with updates after you've done that but that is just a hunch.
So I hope that this info will help you all. Have a nice sunday and happy computing!