Method of Salaah

Imaam starting the qiraat while one is reciting thana

Q: In Prayer behind the Imam, if the Imam starts reciting whilst one just started reading the Thana and he was in the middle of saying اللهم in the Thana but the Imam had already started so he just finished saying Allahumma then he stopped reading the Thana, 

1. Would the prayer be valid? 

2. Would he have missed a Wajib and have to repeat the salah for missing a Wajib?

Imaam making a mistake in qiraat and remaining silent for more than three subhanallahs

Q: The imaan was reading Surah Sajdah. He got stuck repeatedly and paused for more then 3 subhanAllahs. Does salaah need to be repeated because he didn't make sijdah sahw? 

He got stuck like this before. He seems to be ignorant of the masla. 

He also stands with his legs far apart like a ghair muqalid. What should be done in this situation? How should he be corrected?