Is it permissible to daven Maariv/ make Kiddush on Friday after the Plag of the Alter Rebbe/ Baal HaTanya, even though it is before the plag of the Magen Avraham?
No problem to daven after the plag of the Alter Rebbe. Firstly many poskim also held like him e.g. The Vilner Gaon, Aruch Hashulcan, Chazon Ish, Brisker Rov. Also many communities did not keep the time of the Magen Avrohom. Thirdly it seems that the Magen Avrohom only spoke about the time for saying Shema, but as far as minchah and plag haminchah etc could be he agrees with the other poskim.
Magen Avrohom O:CH 58:1 &233:1, Aruch Hashulchon 58:14, , Kitzur Shulchen Aruch 17:1 & 69:2, Chazon Ish O:CH 13:4, Teshuvas Igros Moshe O:CH 1: 24, Teshuvos Vehanhogos 1:56, Orchos Rabenu 1 Page 52.