Because if it would, then the Rema L’shitoso would have to hold Dogon is never Mutter even in a Sfek Sfeka. He argues with the Mechaber in YD 110. Also see this Schach says YD 110 #25 if something is a Dovor Ossur, a Rov is never Matter it.
“The Rosh is clearly referring to a situation where the wheat from each source was kept separate, but it was impossible to determine what that source was.”
Please copy and paste. Also just because the Rema gets his Heter from the Rosh, doesn’t mean he holds exactly like him.
As far as Shem Echad, please quote the whole words inside and copy and paste.
“It is all one single Safeik of this year or last.”
Not necessarily, you can say the imported wheat was from this year and planted 3 days before Pesach.