What is the guidance between the fact that prolongation (tarsul) in the adhan is a Sunnah, and that its desirability (istihbab) is established by a weak hadith, and your quoting of Sheikh Al-Islam Ibn Taymiyyah's consensus that desirability is not established by a weak hadith, and does desirability fall under the category of "virtues of deeds"?

Scholars have unanimously agreed upon the desirability (istihbab) of tarassul (speaking slowly and deliberately) in the adhan (call to prayer). This is indicated by consensus (ijma'), which is a self-sufficient proof. Furthermore, it is reported from Umar ibn al-Khattab, may Allah be pleased with him: "When you give the adhan, be deliberate; and when you give the iqamah (call to commence prayer), be swift." This does not contradict Ibn Taymiyyah's statement that desirability (istihbab) is only established by a legal proof, because ijma' (consensus) is a recognized legal proof.