Did Sai Baba really attend Dev's Udyapan ceremony? I heard Dev was disappointed and felt deceived.

Yes, Sai Baba did attend the Udyapan ceremony, but he did so in the guise of a Sanyasi. As Chapter 40 details, Baba had promised Dev he would come with two others. He then appeared as a Sanyasi with two companions, and they all ate the meal. However, Dev did not recognize him and was clouded by a 'veil of Maya.' Feeling that Baba had not kept his word, Dev wrote a letter to Jog expressing his disappointment. Baba later explained to Jog that he had indeed been there, fulfilling his promise exactly as he had said, but Dev had failed to recognize him.