Does "ag an litir" need to come before "ar mo phósadh"?
Yes. Ag an litir is a local modifier (i.e. where?), ar mo phósadh is a modal modifier (i.e. how?), and local modifiers come before modal modifiers.
Why can't you use éifeacht?
Indeed. And perhaps we can. Also, the meaning of tionchar might be better given as "impact".