1) In terms of all the daily nawaafil prayers, are they supposed to be done in units of two like we do for salaatul layl? For example Maghrib nafila, am i supposed to do 2 + 2 rakats, or one 4 rakat prayer?

2) Is it mustahhab to split the 5 prayers a day into their respective times? Like praying dhuhr at dhuhr time and then ‘asr at ‘asr time? If so, do you know of an accurate website or app that calculates these separated times?

1) Correct

2) Yes it is, the best time for Asr is usually halfway between Zuhr and Maghrib (so around 2 hours after midday in the winter and about 4 hours after midday in the summer), whilst the time for Isha is around 1 and a half hours after Maghrib most of the time