heres another one: saying elokeinu velokei avoseinu (duchan replacement) on fast days during mincha when mincha is davened early in the day. the rama and mishana brura explicitly follow the mechaber not to say it yet everyone does. why?
That is incorrect. They do not explicitly say not to say it when davening early Minchah.
See Chazon Ish O.C. 20 for the reason:
http://www.hebrewbooks.org/pdfpager.aspx?req=14336&st=&pgnum=58