Answer
(Fatwa: 1425/1255=B)
If Mumbai is not your native place, but you only stay there then you have to pray two rakat instead of the four-rakat obligatory salah.
(2) Yes, the salah of muqeem muqtadis (those who followed the imam and were not musafir) will be invalid. It is wajib (necessary) for you to announce it so that muqtadis of the Zuhr Salah repeat their salah. Though you do not remember the date and day, but then also you should announce it so that they make up with the intention of that day.
(3) Yes, you will remain musafir, watan-e-iqamat (the place you stay and which is not your native place) comes to an end by journey; so when you will return there with the intention of staying 15 days or more then you will be muqeem.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband