Is the prostration of the Imam after the Taslim and before completing the deficient Rak'ah considered valid, by analogy with the Hadith of Dhul-Yadayn, or must he first complete the Rak'ah and then perform the prostration of forgetfulness?

What you did is correct, as it is apparent from the words of the Maliki jurists that you descend until you reach the position of rising from prostration only, then you say takbir (Allahu Akbar), and perform the second rak'ah of the Tarawih prayer. Then, after the salutation, you perform the sujud ba'di (post-prayer prostration for忘れ), provided you are not afflicted with waswas (obsessive whispering). If you remember that you missed one prostration, you sit to perform it from a sitting position. However, if you remember that you left both prostrations after standing up, you perform them without sitting. Your prayer is valid, and there is nothing that invalidates it.