Did Dev realize that the Sanyasi he fed was actually Sai Baba in disguise? How did he feel afterwards?

No, Dev did not realize it was Sai Baba. According to Chapter 40, even though he personally served the Sanyasi and his two companions, a "veil of Maya" spread over his mind. He remained doubtful and simply thought of them as uninvited guests who came for a meal. This state of delusion persisted, and after the event, he felt confused and disappointed. He was so uncertain that he wrote a letter to Jog seeking testimony, questioning why Baba had seemingly deceived him and why he didn't receive the profound experience he had been waiting for.