So this is a large project which is good. The larger the project, the more alternative options have to be considered and the direction of progress be for a reason. I'm not saying small projects do not warrant consideration of several options, just that on a larger project the more options that are discarded, the more the final design feels validated and the client feels more they are being presented with the right result for the right reasons. It's their money after all.
Anyway, here's an incredibly generalized view of the process (forgive me!)
- consider more than one layout/design
- look at how each option solves particular needs/problems or what problems it introduces
- adjust/discard the design and iterate
So there is basically an audit trail - a reason at each point why you did something and why you did not do something.
Your DE has a perfectly valid input at the step 1.
At step 2, your ideas and the DE's ideas (and any other stakeholder input) gets considered, which is when you bring up the papers you have in front of you, and importantly your DE also gets to give their reasoning and input - and any papers she has in front of her (?).
If the DE is not UX as you say, then you should find that the DE's influence lies more on the side of input to step 2 and that you're influence lies on the side of output from step 2. If you are right, you should be able to explain why your output best solves the requirements of her input. Likewise the DE.
By step 3, the natural order of things should have directed you a bit further in the right direction, maybe with some of the DE's ideas, maybe not. Complete stalemate usually requires introduction of other people - maybe like the users themselves!