Is one allowed to learn Torah out loud while waiting for a complete Minyan after Yishtabach? If yes, according to who?
One is allowed to learn Torah while waiting for the Minyan to complete after Yishtabach. From the words of the Aruch Hashulchan it appears that one can learn also out loud. (See Shulchan Aruch and Rema Orach Chaim 54, 3, and Mishnah Berurah 6. Dagul Mervavah on the Shulchan Aruch here. Aruch Hashulchan Orach Chaim 53, 4, and 54, 2 & 4). In responsa Beer Moshe (3, 20) he says that when this case occurred by him he learned by thought but not out loud as he was concerned with the opinion of the Magen Avraham (54, 2) who questioned the custom of saying Shir Hamaalot Mima'amakim after Yishtabach during Aseret Yemei Teshuvah. The Aruch Hashulchan however (54, 2) points to this Magen Avraham and says he doesn’t understand why not say the Shir Hamaalot. In addition there is no proof that the Magen Avraham was concerned with the Hefsek after Yishtabach, it is possible that he opposed the custom of saying Shir Hamaalot for other reasons which may be related to the Nusach Hatefillah. Responsa Yabia Omer (2, 4) discusses this issue at length and rules that one may learn out loud between Yishtabach and Yotzer but it is preferable to be stringent and learn by thought only.