Needlessly insulting a deceased person out of anger or ridicule is different from giving reliable information when necessary to establish someone’s rights. Guarding one’s tongue does not mean concealing facts required for justice; it means speaking truthfully, respectfully, and for a legitimate purpose.
In قَدْ أَفْضَوْا, قَدْ before a past-tense verb emphasizes the completed event. In مَا قَدَّمُوا, مَا means “what they sent ahead,” referring to the deeds and consequences that have preceded them.