The helmet is now on the shrine behind the lich. When you pick it up, it disappears (mapchange). If you try to leave without it, you get a very big hint.
This is still not perfect. I could not figure out any way to change Talion's or Ulifendor's dialogue without a huge amount of work (the logic in the dialogues and the three interleaved quests is very bad, and in some cases only works because one mistake cancels another. I do not want to mess with it). This means that if the player kills the lich and still leaves without the helmet, despite the hint, the dialogues still make no sense. That is much less likely to happen now though.
I have tested this, and everything seems to work as desired.
There are also a number of other typo fixes, as reported on the forums, and I made the GoW extraordinary (as also reported on the forums).