Sprint duration
Scrum guide states that "Sprints have consistent duration" but then there is no said rule that sprint duration should be fixed. It sounds pretty conflicting.
I am not following your question, it may be helpful to restate what your conflict is? Wouldn't a consistent Sprint duration imply that the Sprint length is fixed for a certain period of time? A Scrum Team may inspect and adapt the Sprint length after trying it for a period of time, or come to learn that market conditions reveal a change is needed. But changing the timebox of the Sprint often is not helpful to the Scrum Team.
Think about this, how does a consistent Sprint length help the Scrum Team? Might it help the Development Team learn how to deliver a "Done" Increment in a steady rhythm of delivery throughout the release cycle? Might they inspect and adapt their work and processes to deliver within the timebox?