GAON: “I am aware that Rav Ahron accepted the Mishkenos Yaakov regarding Mechitzos in opposing the Bais Efraiyim (as per his responsum in Mishnas R”A) , but where does he pasken like the Mishkenos Yaakov regarding Shishim Ribo?
That is going against all previous poskim, like Magen Avrohom, T”Z, and even the MB leHalacha.”
Rav Aharon states (MRA, 6:10) that that the MB maintains that the majority of Rishonim do not accept shishim ribo, and therefore, Rav Aharon argues the heter is mefulash umechavanim (but then he came up with a chiddush in how we apply this criterion). This is in opposition to the MA and Taz (he obviously understood that the MB was really machmir). However, since Rav Aharon is following the Mishkenos Yaakov, it is understandable.
“Especially if there is a Tzuras Hapesach involved, many poskim uphold that its in any case (even with 600K) not a Reshus haRabim M’Doraisa and is only D’Rabonon that a Tzuras Hapesach won’t suffice…(see Shulchan Aruch Harav 364, Avnei Nezer CM 107)”
Rav Aharon followed the MY who maintains that we require delasos even in a karmelis. However, you are correct, the majority of poskim upheld that a tzuras hapesach would be sufficient on a Doraysa level, and that we only require delasos if all criteria of a reshus hrabbim are met.