Shaykh Nasir aal Mut’ib has published what I found to be the most up-to-date and in-depth defense of the attribution of the Radd to Imam Ahmad in his book, Daf‘ al-Shukuk wal-Awham. Moreover, as a separate addendum to the book, he published a separate piece that addresses further arguments disputing the ascription of the book by contemporary critics, including the likes of Andrew G. McLaren.
Shaykh Nasir’s research is original, meticulous, and detailed. After reading his research, I believe there is no serious doubt remaining that disputes the ascription of this work to Imam Ahmad. Anyone who doubts the ascription and has not engaged directly and sufficiently with Shaykh Nasir’s points should not be taken seriously.
Recommended Reading:
Assalamu 'alaykum, apparently these links are no longer properly functional