From the very moment the party enters Baldur's Gate; they prematurely know about Oskar being possessed, the password to Mystic Carrion's door and the letter involving the Mystic Carrion.
The party should not know the password nor about the Mystic Carrion working with Oskar, until they find the letter in Lady Jannath's estate.
*As can be seen the party prematurely knows everything about the quest's plot*
When the party comes across Lady Jannath at her estate, they immediately know about the letter involving the Mystic Carrion.
The party should not know about the letter until it's found in the atelier's hidden room.
*As can be seen the party prematurely knows about the letter and who sent it*
If the party already came across Thrumbo and chose to help him against the Mystic Carrion, the party will act as if they never met him when they talk to his buddy Lumbar.
The party should not be oblivious to Thrumbo's whereabouts and be forced to pass checks, as they already talked to him and decided to help against the Mystic Carrion.
*As can be seen the party acts as if they never met Thrumbo and want to bring him back to the Mystic Carrion*
