1) After coming from sajda we allahu akbar and then Astaghfirullah but if we say alhamdulillah instead of Astaghfirullah by mistake, then should we perform sajda e sahw?

2) If we miss the wajibs in namaz like niyat, takbeeratulehram, ruku and sajda and we got to know in between namaz that we have left something of these, then can we break the namaz? Or shud we complete the namaz and then again we should perform?

1) It’s fine.

2) If you have missed let’s say ruku while you are in Sujud or about to go to Sujud, you can go back and do ruku and in the end do Sajidah sahw but if you remembered in the second Rakaa that you missed Sujud or ruku you have to repeat your salah, you stop your prayer and resume again