Answered by Shaykh Faraz Rabbani
1. If one is praying nafl salah before dawn and dawn comes in, should one continue as normal or hurry to finish?
Walaikum assalam wa rahmatullah,
One finishes it as normal. It does not count as the sunna of fajr, though. [Imdad al-Fattah]
2. Is it superior to wake up for tahajjad with plenty of time before fajr and when fajr comes in pray and then sleep soon after or to wake up for tahajjad a few minutes before dawn and try to stay up till sunrise.
One should do that which one can sustain long term, and feels most presence of heart and soul with Allah in. If you can do it, wake up for fajr AND stay up till sunrise. Get some sleep during the day, and sleep a little between tahajjud and fajr if you can manage it.
And Allah alone gives success.