I've been wondering, what makes muffins less sweet when using oil? (instead of butter?) In this case, a muffin mix tells me to use butter. The alternative (to make "american style" muffins) is to use (sunflower)oil. It would make it less sweet. 
It is possible for mold to form on cashews - or any other nuts - but only if there has been moisture penetration into the container. If the moisture is at a safe (low) level, then mold won't grow.  See for example, Mycology and spoilage of retail cashew nuts, which refers to the maximum acceptable moisture content of 5.8% for retail storage/shipping, although if you look at their data table, it appears that you may still end up with non-trivial amounts of mold in the low 5% range (which is why you are supposed to store nuts sealed and in a cool, dry place).  Honestly, cashews are hard enough such that you would almost certainly see mold on the surface if it were present in harmful quantities. Most likely what you're smelling/tasting is simply oxidation of the fats (the process which causes rancidity) without actual full-blown rancidity. Mold requires moisture but all oxidation requires is light and maybe a little air exposure.  If you really want to be on the safe side - e.g. if your home is particularly hot or humid - then store your nuts in the refrigerator or freezer (sealed, to prevent contamination or off-odours). Although most (all?) nuts are considered shelf-stable, they do keep longer in the fridge or freezer. 