Grewgious knows more than he shares with the other characters. And he wanted the ring to preserve the memory of his love for Rosa’s mother.
Grewgious is ticked that Drood did not take pursue the girl (who looks soooo much like her mother) while Drood has the opportunity and the means.
Jasper was not a loving uncle, but he has problems that are not quite private.
He is being set up. Others know about his love for Rosa and his opium problem.
The whole town is dark, corrupt, and full of mystery–why not spread the distrust to more than the obvious character?
Jasper doesn’t find Drood’s watch and shirtpin; Crisparkle does–for good reasons.
Rosa does not marry. She realizes that she, in fact, loves Edwin.
She spends her days in the nunnery as she tries to recover from Jasper’s rape. (Come on, he meets her in a garden and makes advances against her will. He penetrates her space emotionally, if not physically.)
Princess Puffer breaks Jasper because he does not know what happened, most likely because of his hallucinations. She blackmails him until he believes that he has committed the crime.
Durdles? Well, Durdles finds the body, but I’m not sure who takes the blame (although it is clear to the reader who killed Drood).
Jasper may take the blame (with the suggestion given to him by Princess Puffer), but he is never sure of the events of that night.