Answer:
(Fatwa: 437/348/H=1431)
(1)Yes, it was right, as the salah you offered with imam was nullified, and his salah was nullified, yours also became invalid.
(2)It is enough to say: ربنا لك الحمد
(3)There is no harm if read in heart.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband
