Answer
(Fatwa: 669/561=D/1430)
(1) According to the Hanafi Fiqh, it is sunnah to say ?Amen? silently, so saying it loudly will be against sunnah.
(2) One can recite the dua in nafl prayers.
(3) According to the sayings of Zakir Naik quoted in people’s questions, he is religiously deviated, some of his talks are unauthentic. A common man may not be able to differentiate between right and wrong; therefore people should avoid listening his speeches, they are feared to fall in deviation.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband