Salaah

Performing two rakaats sunnat before zuhr due to the fardh salaah commencing

Q: I was offering the four rakat sunnat before zuhr in the mosque. I was offering the second rakat when the iqaamat started, so I completed my salah like a two rakah prayer to join the jamaat. Will I have to offer the sunnat again after zuhr or is it permissible to skip it? And also should I do sajdah sahw when I repeat the sunnah to after the jamaat?

What should a masbooq recite when standing up for missed rakaats?

Q:

1. Suppose I am offering Asar or Maghrib and I joined the Namaz in the last Rakaat after missing the first three rakats, now when the prayer ends with "Assalaamu alaikum warahmatullah" by Imam and I get up from "Attahiyyaat", Then what do I have to say first? Do I have to start my next rakat with thana or bismillah?

2. Also, do I have to say a surat after reciting surah fatiha?