Is it permissible for a person to pray Qiyam al-Layl sitting down after having prayed Witr, and how did the Prophet, peace and blessings be upon him, perform ruku' sitting after Witr?

The correct view is that the Prophet's two rak'ahs (units of prayer) after Witr, performed while sitting, were to show the permissibility of praying after Witr and the permissibility of performing supererogatory prayers while sitting. He did not perform them regularly; rather, he did them only a few times. Interpreting the hadith (definition) in this way aligns with the numerous narrations that indicate his last prayer at night was Witr, and with the hadiths commanding that the last prayer of the night be Witr.