What does "mission mixing" mean for long-range aircraft?
Mission mixing appears to be running computer simulations (or other probabilistic analyses) to assess aircraft engine components' reliability also under non-typical or non-designated use. All for the purpose of design or maintenance/inspection strategies, I would assume.

So, mission mixing is not something you would purposely do with actual aircraft. (Or maybe you would, but then it is not called mission mixing.)

Here are a few articles mentioning it:

https://arc.aiaa.org/doi/pdf/10.2514/1.13142
https://asmedigitalcollection.asme.org/GT/proceedings-abstract/GT2012/44731/439/247656

http://en.cnki.com.cn/Article_en/CJFDTotal-HKXB902.017.htm
https://www.astm.org/DIGITAL_LIBRARY/STP/PAGES/STP14812S.htm