Did Sai Baba actually attend Dev's Udyapan ceremony, and if so, how did he appear?

Yes, Sai Baba did attend Dev's Udyapan ceremony, though Dev failed to recognize Him at the time. According to the account in Chapter 40, Baba fulfilled His promise by appearing in the guise of a Sanyasi. He first came alone, causing Dev to worry he might ask for money. Baba then assured him He would return for the meal with two others. True to His word, the Sanyasi returned at noon with two companions, and all three were served a satisfying meal. Baba later confirmed this to Jog, stating, "I said I would come with two others, and I went with two others," emphasizing that He never breaks His promises.