بسم الله الرحمن الرحيم
(a) Yes! A Shariah musafir may lead salah but he should turn salam on two rakah in salah of four rakah. After he turns salam the muqtadis will complete two rakah, they will not read surah Fatiha, rather will stand silently equal to the stay of imam then they will bow for ruku. However, here it should be noted that if the gathering is not of educated people then Shariah musafir should not be appointed as imam. However, if he is made imam due to some reasons then before starting Salah or after turning salam he should announce that he is musafir: O people complete rest rakahs.
وصح اقتداء المقيم بالمسافر فى الوقت وبعده فإذا قام المقيم إلى الإتمام لا يقرأ ولا يسجد للسهو فى الأصح … وندب للإمام … أن يقول بعد التسليمتين فى الأصح اتموا صلاتكم فإنى مسافر (در مختار مع الشامى 2/611، ط: زكريا)
(b) If musafir imam leads four rakah then the salah of muqim muqtadi will not be valid. As far as the salah of imam is concerned then it will be valid with karahat provided he has done first qa’dah but it will be wajib to reoffer it within time. If time has passed then it is not wajib to reoffer rather it is mustahab.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.