Answer
(Fatwa: 1235/1235=M/1430)
(1-2) If the muqtadis are comfortable and the mosque is spacious, the imam can lead salah standing in the middle of the first row leaving the mihrab. (Fatawa Mahmoodiah) In this case, the muqtadi should not unnecessarily blame the imam. Yes, if the imam does not stand in the middle of row, his attention should be drawn to this point; since standing in the middle is sunnah.
(3) It is makrooh to perform salah in banyan in spite of availability of kurta or shirt.
(4) The salah will be valid if the satr (parts necessarily coverable) are covered.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband