So basically though linear="no" is in epub2 spec no commonly owned physical ereader or commonly used iOS or Android app actually uses it, as per the 2017 thread.
It's irrelevant if Nook or Kobo supports it tomorrow. Most users won't have it. Readium is almost irrelevant market share.
So don't use
Code:
<itemref idref="c1-answerkey" linear="no"/>