[RPG] Can a Troll survive being decapitated

Though the PC's in the situation described won’t die straight away, the astral plane is a very dangerous place. I would refer to the DMG on pg: 47

A character who travels bodily to the Astral Plane (by means of the plane shift spell or one of the rare portals that leads directly there) has no silver cord.

and when using astral projection

Only the severing of a character’s silver cord while on the Astral Plane (or the death of his or her helpless physical body on the Material Plane) can result in the character’s true death. Thus, high-level characters sometimes travel to the Outer Planes by way of astral projection rather than eek out a portal or use a more direct spell.

Basically this means that they will be transported “bodily” and therefore are at the mercy of everything the astral plane can throw at them such as psychic wind and githyanki.

So though they don't die en route, they will likely die there unless they find a colour pool for the prime material plane

(spoiler alert)

The colour for the prime material plane is silver as stated on pg 47 of the DMG.

The troll turns into a dead pile of fine gray dust

The disintegrate spell reduces the troll to a pile of fine gray dust. The pile of fine gray dust is no longer a troll and therefore does not have the regeneration trait (for the same reason a troll polymorphed into a frog also does not have the regeneration trait). In case there was any doubt that the pile of fine gray dust is dead, Jeremy Crawford explicitly states that disintegration is death in this Sage Advice column:

If disintegrate reduces you to 0 hit points, you’re killed outright, as you turn to dust.

(Note: I believe that the troll's regeneration ability does not give it immunity to instant-kill effects, for the reasons outlined in this other answer, but the rationale I've given here for the specific case of death by disintegration is independent of that answer.)