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Sorry, I dont know any keys for determination of flies from pictures. If you prefer English publications you might use Google for search. Here you will find for example:
https://diptera.info/downloads/StN_KEYS_Glasgow_2011.pdf or http://prodinra.inra.fr/ft?id=6E1CD73C-EFBD-4BC9-A04C-452A3F646C46 which you can find at the download area of this side: https://diptera.info/downloads/StN_Species_Accounts_Glasgow_2011.pdf.
A paper which might answer some of your questions as well is: Speight M.C.D. & Sommaggio D. 2010. On the presence in Switzerland of Microdon myrmicae Schönrogge et al., 2002, Xanthogramma dives (Rondani, 1857) and X. stackelbergi Violovitsh, 1975.
Entomo Helvetica 3: 139–145.
Pleural spots are quite variable at least in X. pedissequum and X. dives as far as I remember from specimens I examined.
Regards
Piet