Answer
(Fatwa: 1064/1064=M/1430)
If the beliefs of the imam are not amounting to shirk (associating partner to Allah) and he bears qualities necessary for imamah (leading prayer), one can perform (five-time and Eid) salah behind him.
(2) There is no need to perform the Witr Salah anew, rather when you remember while reciting dua or durood, you should turn your head for salam and perform sajdah sahv (remedial prostration).
(3) One should say Al-Hamdulillah as praise to Allah, Ma-sha-Allah is said to express admiration and surprise, while JazakAllah is called as dua for one who did a favour to you.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband