Is one whom God guides to know the interpretation of the disconnected letters in the Mushaf (such as: Alif, Lam, Mim; Kaf, Ha, Ya, 'Ain, Sad) obliged to inform people of it, or does his turning away from it not make him sinful?

The interpretation of the disconnected letters (al-Hroof al-Muqatta'ah) must be in accordance with what the Salaf interpreted them to be. It is permissible for whoever has been guided to one of their sayings to teach it to the people. As for a new interpretation that none of the imams of tafsir (Qur'anic exegesis) have ever put forth, it is not considered valid, and one must turn away from it.