You forgot to mention that the XDNA passed on by Mom is recombined from ther two X's
As I understand it XDNA tracks differently than mtDNA, and can be used to eliminate spurious matches. If I recall correctly 23andme does have X DNA matching, as well as auDNA matching. In other words XDNA matching is more narrow than auDNA, just as YDNA is more narrow than auDNA.
As a male I have recombined X DNA from my mother, as well as au
DNA from Mom and YDNA from Dad as well as auDNA from Dad.
Mom got her Dos Equis from Mom and Dad, Dad got a recombined X from Mom and a Y DNA from Dad.
Mom's recombined XDNA contains her Dad's unrecombined XDNA and her mothers recombined XDNA.
It seems to me that tracing this X DNA, as opposed to auDNA, would provide a more narrowly defined, and thus more accurate, match.
Alas, Family Finder does not provide X as well as auDNA.