Is it necessary to unvocalize the emphasized letter (mushaddad) when reciting the Holy Quran, such as in "الضالّين" (al-ḍāllīn) to become "الضالْلين" (al-ḍāl-līn), while it is not unvocalized in words like "لعلّكم" (laʿallakum)? And is there a rule for this?

It is not permissible to break up a doubled letter (mushaddad) in the Qur’an; rather, it must be pronounced as a single, doubled letter. Correct recitation does not depend solely on what is written, but must be taken from the mouths of proficient reciters, as recitation is a followed tradition (Sunnah muttaba‘ah). Therefore, it is advisable to accompany one of the proficient shaykhs to receive the recitation directly from them, as the rules of Tajweed cannot be mastered except through direct reception.