I didn't mean that free will itself was trivial, but that in the explanation you offered it seemed secondary to the primary necessity of an "adversary." Thus "trivial" as opposed to "overarching." I suppose "secondary" was the word I actually meant and should have used.
No, that isn't my meaning at all. Satan plays a part in the plan as a tempter; one who prods, entices, provokes, goads, incites, and instigates evil and rebellion against the Plan. The spirit of the Lord attempts to pull us in the other direction. This all takes place embedded within the overarching background of free agency. We are free to respond more to one than to the other. We are free to make choices regarding which influence to follow, against a range, in various circumstances, of possible alternatives.
There isn't a hierarchy of importance here, to the various elements of the Plan. They are all a part of a whole; they are all interdependent and interconnected. The Book of Mormon explains that it is not enough that we just have our fallen, mortal nature, with its inherent tendencies to selfishness, self aggrandizement, and carnality, over against the equally natural tendency to do good, to be concerned for others, and to resist the weight of our fallen natures that we inherit from our eternal Father. There must be an influence exerted on one side or the other such that our choice making in this life becomes much less passive. Pressure is exerted upon us to move one way or the other.
This is a major teaching of the Book of Mormon, and one of the "plane and precious" truths somewhat lost to post apostolic Christianity.