So we know that the parasite leaves the host if the host dies. We see this at multiple instances.
Now every character starts with a Scroll of Revivify. Why can’t we just kill the characters, wait for the parasite to crawl out and revive them directly after?
I know of course this would stop the game to progress and the whole plot would stop to work… but this little plothole annoys me in a funny way :-D
We can explain it with ingame mechanics and “lore”. The parasite only leaves the body once the body is beyond repair. In 5E revivify only works within 1 minute of a characters death. But the parasite itself is what allows us to use exceed that limit. And we see that with companions, if they are dead dead and you remove the parsite from them, you can’t revive them anymore. Not with a scrolls of revivify nor Withers. It’s not perfect because technically the parasite could keep the body revivable for decades but your character also wouldn’t age.
Only with True Ressurection or a Wish Spell things would get more difficult to find mechanical/lore reasons why this method doesn’t work. But even there you could just say that the parasite has become a part of the person and would simply be resurrected as well.
And Withers?
Withers doesn’t change anything. He too can’t bring back a dead companion that had the parasite removed after death.