Hi there. I wanted to ask, if its allowed to speedrun the Sand Hill through the "Trial", and then add timer via the video editor? Or it is necessary to do it in the Story mode when you chase Froggy?

Level LB uses the in-game timer, so you must use Story Mode.
That being said, there is a collection of save file for Tails's Story in the resources, you will find a save file just before Sand Hill.

https:/​/​www.​speedrun.​com/​sadx/​resources