Regarding your questions, the first answer is that it is correct that the body transcendental of Krsna and Krsna Himself are nondifferent. So what is the difficulty to understand that the soul of Aghasura merged into the body of Krsna? In other words Krsna benedicts the demons Whom He kills personally with the impersonal liberation of merging with Him.
Letter to Ekayani Dasi, 25 July, 1970