Conditions for the validity of Salaah

Beginning of Esha time

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.

Doubts regarding a leather belt

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? 

Intention of different nafl salaah when performing one nafl salaah

Q: I came to know by one of the dawat-o-tabligh elders that we can make intention of many different namazes (e.g: if we are praying ishraq then we can make intention of tahiyaatul wudhu, tahiyyatul masjid, salaatul hajat, salaatul taubah, etc.)

1. Is it true?

2. How many total extra niyyats can we make for a two rakat nafil?

3. Can we do the same for zohar, maghrib and isha nafl prayers?