How did the Prophet, peace and blessings be upon him, recite the morning and sleep remembrances (adhkar)? And did he recite all of them or just one supplication (du'a)? And did he sit after prayer to recite all the remembrances?

We have not found a clear transmission regarding this specific issue. However, Sheikh Al-Uthaymeen holds the view that the Prophet (peace be upon him) used to vary between multiple supplications that shared a single cause, in order to fulfill the Sunnah in all its aspects, to make it easier for the Ummah, to ensure mindfulness, and for the Muslim to comprehend what he is saying. Ibn al-Qayyim mentioned in "Zad al-Ma'ad" that when the Prophet (peace be upon him) finished the prayer, he would seek forgiveness three times and say: "O Allah, You are As-Salam, and from You is peace; blessed are You, O Possessor of Majesty and Honor." He would then turn to his right and to his left, and after every prescribed prayer, he would say: "There is no god but Allah alone, He has no partner. To Him belongs the dominion, and to Him belongs all praise, and He is over all things competent. O Allah, there is none who can withhold what You give, and none who can give what You withhold, and the fortune of the fortunate does not avail against You." He would also say: "There is no god but Allah alone, He has no partner. To Him belongs the dominion, and to Him belongs all praise, and He is over all things competent. There is no might or power except with Allah. There is no god but Allah, and we worship none but Him. To Him belong all blessings, and to Him belongs all grace, and to Him belongs all good praise. There is no god but Allah, sincere in devotion to Him, even if the disbelievers detest it." He also encouraged his Ummah to say Subhanallah (Glory be to Allah), Alhamdulillah (All praise is due to Allah), and Allahu Akbar (Allah is the Greatest) thirty-three times each, completing the hundred with: "There is no god but Allah alone, He has no partner. To Him belongs the dominion, and to Him belongs all praise, and He is over all things competent."