Shlichus for non-Jew

(See previous post first!) The Machne Efrayim attempts to bring an interesting proof. The gemara in Avodah Zarah 53 asks why the trees worshipped as avodah zarah (asheiros) had to be destroyed by the Jews entering Eretz Yisrael - since Eretz Yisrael had been given to Avraham, and we have a principle that "ain adam oseir davar sh'aino shelo", a person cannot create an issur using someone else's property, how did the trees become assur? The gemara answers that when Bnei Yisrael worshipped the eigel, their tolerance for avodah zarah served to establish that the idolators worshipping trees of Eretz Yisrael were acting on their behalf through "shlichusayhu". The Machneh Efrayim (Shluchin 14) writes that we assume a Jewish apostate is treated in halacha like a non-Jewish idolator, so here we have a source that indicates a non-Jew (those living in Eretz Yisrael) may serve as an agent and shliach for another non-Jew (the apostates who worshipped the eigel)!
The Machne Efrayim is based on a few difficult assumptions: 1) a Jew who worships avodah zarah may be treated l'chumra with certain stringencies that apply to non-Jews, but such an individual is still considered a full fledged Jew; 2) "shlichusayhu" is not the same as agency/shlichus in the formal sense - it just means that the worship of idols was approved of by them (by way of comparison, see the sugya in Gittin 88b. Formal shlichus is a din d'oraysa; shlichusayhu as it applies there is derabbanan, see the rashba).