How can the sending of every Messenger with the language of his people, and the sending of Muhammad with the Arabic language to warn Umm Al-Qura and those around it, be reconciled with the universality of his message to all humankind?

The message of Prophet Muhammad (peace be upon him) is universal for all worlds, not exclusive to Arabs. The noble verses confirm this, such as "And We have not sent you except as a mercy to the worlds" and "that he may be a Warner to the worlds." The fact that the Qur'an was revealed in the Arabic language does not mean the message is exclusive to Arabs; rather, it was in their language because they were the first to receive the message, and the most noble of languages was revealed because it is the final message.