Salaah

Performing the witr before going to sleep

Q: It was the practice of Nabi (sallallahu alaihi wasallam) to read his witr salaah after he slept and woke up for tahajjud. I would like to follow this Sunnah, however there is fear that I may not wake up for tahajjud and miss my witr. 

Is it permissible and will I get the same reward as following a Sunnah of Nabi (sallallahu alaihi wasallam) if I delay my witr till before I go to bed, read my witr and then sleep? Do I get the reward of following Sunnah?

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?