How can one reconcile the two Prophetic traditions concerning the recitation of Surat al-Kafirun before sleep and the supplication "O Allah, I have submitted my face to You..." and making it the last thing one says?

There is no contradiction between the Hadith of Al-Bara' bin 'Azib and Nawfal Al-Ashja'i regarding the supplications before sleep, even if Nawfal's Hadith is deemed hasan (good). This is because phrases indicating "lastness" or "bestness" are often used in the Arabic language to mean "among," such as "among the best of them." The purpose is for these supplications to be among the last things recited. Therefore, whoever concludes their supplications with Surah Al-Kafirun and then recites the supplication "O Allah, I have surrendered myself to You," or vice versa, has achieved the "lastness" and complied with both commands. Moreover, scholars who compiled the daily and nightly adhkar (remembrances) mentioned both Hadiths together and saw no contradiction. This is further indicated by the fact that it is prescribed for one who has finished the adhkar to continue in dhikr until they fall asleep, and they are not considered to have abandoned the Sunnah by doing so. This confirms that what is meant by "lastness" is not an absolute cessation, but rather that it should be among the last things one recites.