Is the pronunciation of the letters "Dal" and "Seen" considered correct when reciting silently if the sound of the Dal comes out with a whistle and the Seen without a whistle? And is it permissible to adopt the view that merely moving the lips is sufficient for silent recitation, and which scholars held this opinion?

The articulation point of the letters "dal" (د) and "ta" (ت) is the same: the tip of the tongue with the roots of the upper incisors. Pronouncing the "dal" in this manner is correct. As for the absence of the "safir" (whistling) quality in the "seen" (س), it does not affect the correctness of the recitation nor does it invalidate the prayer. Reciting without the reciter hearing themselves is not sufficient according to the majority of scholars, though some scholars have stated that merely moving the tongue is sufficient. It is recommended to ignore whisperings (waswasah) regarding the articulation points of letters, as they are among the devil's insidious ways.