What is the time given for earliest Shema?
In most places in Israel (and some places in America) there are two times listed for Tzais Hakochavim. The earlier time is for the earliest Shema and the later one is for the time that Shabbos ends. In most places in America they did away with the earlier time in order to avoid confusion, but you are allowed to say Shema earlier than when Shabbos ends and it is considered Bizman. According to Rav Schachter, the earliest time to say Shema is Tzais according to Rav Schneuer Zalman of Liadi, which is 24 minutes as degrees after Shekiah.
Why can the 4:20 Glueck 2 Minyan do Heichi Kedusha?
Originally the Minyan Davened Heichi Kedusha, but they switched to a full Shemoneh Esrei a few years ago. However, now when the Chazzan used to finish Kedusha a number of people would leave and we would lose a Minyan of people answering for Chazaras Hashatz. Therefore, in order to make sure that we don't lose a Minyan, the 4:20 Minyan can do Heichi Kedusha, as per Rav Schachter.