Missing Two Rakaats Salaah

Q: We entered the masjid for Isha namaaz. We unfortunately missed 2 rakaats and therefore joined in the third. The Imaam then incorrectly performed 5 rakaats, and the Musalis decided to remake the Isha namaaz. Were we to complete just one rakaat, as we performed 3 behind he Imaam or should we make salaam after 1 rakaat and re-join the Jamaat?
Also, is it correct in saying that if you join the Jumuah salaah after Ruku in the second rakaat, then you have to perform 4 rakaats?
A: (a) You should make Salaam after one Rakaat and re-join the Jamaat.
(b) If you join the Jumu'ah Salaah after Ruku in the second Rakaat, you have to perform two Rakaats and not four.
Prepared by: Moulana Yusuf Laher
Checked and approved by: Mufti Siraj Desai