Well (tee-hee), there is precedence for those - at least one precedent that I know of. The only parallel sevenths I have encountered are in the first verset of the Kyrie Cunctipotens in Pierre Attaingnant's Tablature pour le jeu d'orgue... of 1531....descending 7ths...
It takes years to get good at the art. Don’t let that discourage you. Join the order of eternal students.Choral writing such as this requires attention to several layers of detail: voice-leading, harmonic progression (and harmonic rhythm), interrelation and balance of the separate voices.
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