Conditions for the validity of Salaah

Q: Is wearing a skull cap mandatory whilst praying Salaah?

Q: I had offered 2 rakaahs of sunnah after zuhr, but I forgot that I already offered it. So I offered it again, but I mistakenly faced the wrong direction instead of the Qiblah. Should I offer the sunnah again?

Q: Till what time can one offer fajr prayer? 

Q: Just want to find out which salaah's can be joined (read at the same time) when one is a traveller?​

Q: I read that Maghrib prayer time ends when the redness in the sky goes away. Also I read that it ends half an hour before Isha Azan. Which one is true? Half an hour before Isha azan I feel the sky becomes darker. Can I still pray Maghrib or should I mention as qadha namaz? As I'm a working woman and gets little late to reach home and can't pray in van.

Q: Can we perform salaah with clothes on which has animals like Puma shirts, etc.?

Q: If fajar azan is at 4:45 am then what's the maximum last time tahajjud can be offered?

Q: A lot of people tell me that you can delay isha prayer till midnight, is this true?

Q: If I have filth on my clothes less than the size of a dirham and water touches it and the diameter increases from one dirham, can I pray in those clothes?

Q: I wear a trousers belt, it's leather. I don't know what kind of animal skin was used to make this leather belt. I heard that if you wear leather made with pig skin and if you pray salah with it, it wouldn't be accepted, it's nearly impossible to tell if the leather is pig skin or not because where you buy it, it doesn't says. Would my salah be valid?