Conditions for the validity of Salaah

Q: Can I read Fajr namaaz after Azaan without waiting as we read maghrib?

Q: Is namaaz valid in half sleeve shirt?

Q: What is the definition of twilight? In beheshti zewar English it says the redness on the western horizon, when that disappears then esha time comes in. However some kitaabs mention the twilight according to imam Abu hanifa as the whiteness after the redness. Which is correct. Why is it mentioned as redness in the beheshti zewar, would this not make our esha time early like the shafis etc.

Q: What is the definition of sunrise/shurooq? When the sun slightly appears on the horizon or when the sun completely is visible on the horizon?

Q: I performed Salaah not knowing that there was mazi on my clothes. It was only a few hours after performing the Salaah that I realized that my clothes has been soiled. Is that Salaah valid or do I have to repeat it?

Q: Upto how many minutes before Ghurub aftab, Salatul-asr can be prayed and how many minutes before ghuroob aftaab, makrooh time for salatul asr starts?

Q: My question is about differing start and finish times for Salaah. The mosques I have near to me have different times for Salaah. For example one mosque start time for esha is 9:30pm and another is 10:20pm. If I usually pray in one mosque can I follow the timing for the other, again as an example I follow the mosque with the 10:20pm start but offer my esha prayer before that as the other mosque clearly starts at 9:30pm. Is this permissible?

Q: What is the reason behind prohibition of Salaah during Tulu' (Sunrise), Zawal (Zenith) and Ghurub (Sunset) period of the sun?

Q: I am giving exams when I depart from my home. It's Zohar time so I offer it but when I arrive at my home, it's maghrib time so can I offer Asr Salaah at Zohar time or at Maghrib time?

Q: A radio station broadcasts Surah Waaqiya immediately after the Magrib Azaan. Is it permissible to delay performing the Maghrib salaah by about seven minutes and listen to the recitation of the Surah first?