Qiraat in Salaah

Reciting a surah after surah faatiha before going into ruku

Q: I was praying maghrib. After reciting Surah Faatiha in the first rakah I forgot to add another surah and was going for ruku. I opened my arms but just when I said "Alla-"(of Allah Akbar) I realised my mistake. I once again joined my arms and recited the second surah and did rest of the salaah normally. I did not do sajda sahw. Is my prayer valid? 

Also is sajda sahw valid for all mistakes in salaah or is it invalid if you missed fards of salah?

Reciting the first 3 verses only of Surah Faatiha in every rakaah of salaah

Q: As we know it is waajib to recite the first 3 verses of Surah Faatiha in every rakaah of salaah.

I heard a mufti say that qiraat changes the amount of verses in the Quraan. If I recite the first three verses of Surah Faatiha and combine the second and third verse and not recite further would my prayer be valid?