Qiraat in Salaah

Q: Why are zohr and asr called sirri salaahs and the Imam would not recite loudly? Any significance? Please explain.

Q:

1. I have been told that if, for example one reads surah Nasr in the first rakaat, then in the next rakaat one cannot read surah Ikhlaas - it has to be the next surah, or a gap of 2 surahs. Please clarify this rule for me (e.g., does it apply only to short surahs, does it apply to 4 rakaat salaah as well, etc.)
2. For long surahs, can one read from different parts of the surah in consecutive rakaats. Eg., 1st rakaat beginning verses of surah Najm, and 2nd rakaat ending verses of surah Najm.

Q: Question regarding reciting inaudibly. In my case I was praying Dhuhr Fardh and quietly as to hear my own voice, in my last (4th rak'ah) - I was reciting "Ghayril Maghdoubi" at Maghdoobi, I could only clearly hear "Ma--doobi" the Gha wasn't proper so that I could hear, but I moved my tongue to recite it as normal. Obviously I didn't want to recite "Ma--doobi", so I think I inaudibly said the gha, does this matter or affect the validity of the namaz?

Q: When I perform Salaah alone, should I recite Bismillahir Rahmanir Raheem before Surah Fatiha in every rakaat of Salaah? Also, should I recite Bismillahir Rahmanir Raheem after saying Aameen and before I start another Surah?

Q: Assuming in the first rakaat of witr, one recites Surah Lahab and Surah Ikhlaas in the second rakaat. Is it necessary to make sajda sahu as only Surah Falak or Surah Naas can be recited in the third rakaat but these Surahs are longer than Surah Ikhlaas. Likewise, in any other namaaz, if one recited a Surah in the second rakaat that is slightly or much longer than the surah recited in the first rakaat, what is the ruling?

Q: When one is performing Salaah, is it necessary for one to hear his own voice and whatever he is reciting?

Q: I started praying magrib alone before another brother joined me, so during the second rakaat I recited only one verse from the Quran (sura ghafir verse 44) after fatiha and went to ruku position. Since then I have been having doubts if my salaah is valid or not. If not what should I do because that brother is gone now. 

Q: If one omits the Tasmiyah before Surah Fatiha in the 2nd, 3rd and 4th rakaat of Salaah due to habit, will the Salaah be acceptable? When we read Salaah with Jamaat, the Tasmiya is always silent and unfortunately I developed this habit.

Q: Whilst praying maghrib salaah, what should we do if we accidently pray surah falak and surah naas in the three farz rakaats. Do we pray the same surahs in the two sunnat rakaats as well or can we pray any other surahs?

Q: If one has to recite only one Ayah in a rakat of a Salah how long it should be?