The episode is an excellent one, but I disagree that we will see Danny Darrow again. I think he died in the collapse. This is the classic trope of "redemption equals death" since he saw the error of his ways and gave up the Piece to the gang.
Rewatched this one too, and have to say this may be one of my favorite episodes.
This was a truely interesting mystery all around, and felt more like a 'classic' episode in many ways. Maybe because the gang get split up and then run into each other again.
But is was also one of the most scary and riveting eps, as Scooby goes. All the deadly boobytraps they had to get past, and the narrow escapes. And one of the most spooky and creepy mansions we've seen. When the gang was trying to escape at the end between the walls, and Danny was ramming that fireplace poker through the walls trying to stab them, was one of the most riveting scenes yet.
That and the ending that has more of a message than most Scooby eps, and a 'villain' you come to like and feel sorry for by the end was really different.
All in all this was one of the more serious episodes overall we've seen, and sorta got back to the somewhat 'darker' feel of the early SD:MI episodes when the series first started.