Yes, upgrading to 126.96.36.199 is needed before moving to Win10 1709 to avoid multiple known issues, however, I don’t know that it is the cause of your upgrade failures. The other posts Alexa mentioned describe failures occurring even at that level.
We are aware of problems occurring during 1709 upgrade via fixlet, and are investigating, but have not identified a single pervasive issue. As the other posts suggest, some upgrades are successful, and there are different causes when failures occur. Alexa, your issue is obviously pervasive in your environment, so hopefully you have opened a PMR and are working with our teams to identify root cause.