Dev wrote a letter complaining that he felt deceived by Baba. Why did he change his mind so suddenly just because of a letter from Jog? Isn't it possible this was just a convenient story to cover for Baba not showing up?

Dev's change of heart from doubt to remorse was a direct result of the detailed explanation he received. According to Chapter 40, Dev initially felt disappointed because he expected Baba to appear in a way he would easily recognize. He wrote his complaint to Jog, who then presented it to Sai. Baba's response, relayed back through Jog, contained specific details, including Dev's initial fear that the Sanyasi had come for money and the reminder that he would come with two others. Upon reading Jog's letter containing these specific proofs and Baba's unwavering assertion that "the words from My mouth shall never be otherwise," Dev realized his error. His eyes "overflowed with a surge of love," and he felt shame for his lack of faith, not because of a convenient story, but because Baba's account perfectly explained the events he had failed to understand.