Earlier, Rabbi Roi Margolit posted, "One should neither talk nor even study or make up parts of the prayers during the repetition by the Shatz (Shulchan Aruch Orach Chayim 124:7)," but the citation there states that one should not engage in sichat hullin. Is study considered sichat hullin? Or is it that one cannot properly study if one is trying to say amen? What if there are at least a minyan of other people saying amen so that the shatz is not saying a berakhah levatalah, is it permitted to study then?

As you quoted:"One should neither talk nor even study or make up parts of the prayers during the repetition by the Shatz (Shulchan Aruch Orach Chayim 124:7)," - talk includes sichat chulin. There may be a minyan of people who are answering amen - it does not allow you to disregard the Chazarat Hashatz, even Study is disregard and disrespect to the prayer. Rabbi Ro'i Margalit