For those who like me didn't find all answers to essential questions:
1) absolutely all time count once you start: this includes play time, death, and all loading times...
2) quit a level doesn't not restart the countdown (I did it once)
3) re-enter a level already done doesn't affect the trophy, and doesn't reset the countdown (I did it once)
3) you can die (as many times you want
) this doesn't affect the trophy (I died several times)
4) I
strongly guess countdown is set (or reset) when you start from the main menu's game (where you can choose your save slot); I re-start from there a couple of time when i was not satisfied with my start, each new start with a countdown running on my phone and I get the trophy.
5) Trophy unlock after the last loading time of the last achieved level to return to the hub.
6) Order of the levels doesn't matter: I made it from 20 to 1, without any respect of any order.
7) You can use (and abuse) slow motion, this doesn't affect the trophy (I use it on level 16 & 20), however it takes slightly more time to achieve levels
8) I would recommend to do all levels of a section before trying another section, as travel between each section takes time, and coutdown is always running... So choose to start with the section with the most struggling level for you...
Hope this helps someone, one day...