Times of Salaah

What is twilight

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.

Performing Esha Salaah once the time sets in

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?

Combining Salaahs

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?

Performing Salaah in the forbidden time

Q: As I live in a city and because of the buildings, I can't always see the sky openly. How can I determine that time when salah is forbidden? Is there any trustful program to use, do you know one? I hope it is the one that shows time as salah calendar as I don’t always have wifi on at home? And also, if I didn’t make my fard yet and the forbidden time started, can I still make my salah if I see that due to some reasons I will miss it - like toilet is busy, water is going to be off, etc? How sinful is it to make salah at this time? Also, if I start salah some minutes prior to this time and ended it when this time has started, do I have to repeat my salah?