Is the message of the Prophets of Steadfast Resolve universal, and if not, why did the disciples call upon other nations, and who are the messengers of Abraham and Moses mentioned by Ibn Taymiyyah in the context of denying the infallibility of the disciples and their being messengers of God?

The statement that the message of the Messengers of Strong Will (Ulul Azm) was general is incorrect. Messengers before Muhammad, peace be upon him, were sent specifically to their own peoples, such as Jesus, peace be upon him, who was sent to the Children of Israel.

The fact that prophets were sent specifically to their own peoples does not prevent the Messenger or his followers from inviting those outside their people, or from people not of their kin believing in him. An example of this is the call of the disciples (Hawariyyun) and other companions of the Messengers to their peoples and to others, as mentioned in the story of the people of the town in Surah (definition) Yasin.

Every prophet had companions and disciples who carried his call and conveyed it, even if their names were not explicitly mentioned in the Noble Quran, such as the companions of Abraham who disavowed their polytheistic people. Some exegetes (mufassirun) mentioned that messengers of Moses and Jesus, peace be upon them, came to the Arabs.

As for the messengers of Muhammad, peace be upon him, during his time, they are well-known, such as Amr ibn Umayyah al-Damari and Dihyah ibn Khalifah al-Kalbi, among others.

In summary, all messengers of Allah were tasked with conveying the message and inviting specifically their own peoples, and there is no impediment to the Messenger, his companions, or his followers inviting those outside their peoples.

The one who was sent to all people and entrusted his followers with inviting to his law is the Seal of the Prophets, Muhammad, peace be upon him. Allah sent him to all creation, humans and jinn, Arabs and non-Arabs, in confirmation of His saying: "And We have not sent you except comprehensively to all mankind" and His saying, peace be upon him: "And a prophet used to be sent specifically to his people, and I have been sent to all mankind."