Rose did not learn how to play the rain. Playing the rain means she’d have to “produce the musical analogue of a genetic code to reintroduce life into the ocean.”

Jake’s chumhandle golgothasTerror is a reference to the place where Jesus was crucified. 

Terezi’s computer.

Already answered. 

Jade explains here. 

Here are some unsolved plot strings. We also don’t know the full timeline of the Warhammer of Zillyhoo, who sendificated the last frog, or who put the weapons in the Lotus Flower Time Capsule. Those are just the ones off the top of my head. 

The black forms for the adult trolls is just a stylistic approach, just like the human guardians don’t have all of their face’s depicted. Also, all the troll blood colors have been revealed in-canon. 

If they’re dream selves were still alive then they most likely would retreat into those bodies. 

The details of Mindfang’s death were never revealed in-comic so all we know is that he killed her. In the fan animation After Us by toastyhat Mindfang controls the Summoner to kill her to put her out of her misery when she was injured and dying. That’s one way to explain her death. We just don’t have enough information in-canon. 

Jane’s trollsona design was based off of a drawing by Shelby Cragg, who is the artist that does Calliope’s drawings.