There aren’t multiple Green Suns, only one. Also each planet has a First Guardian, not each session. 

Aranea says here that their psychic abilities, the vision eight-fold, comes naturally to her blood caste.

Up to your interpretation! Much of the fandom thinks that the juju trapped Lord English in the Medium, others think it destroyed him. We might not find out for sure until the epilogue. 

Meenah was trying to articulate that it doesn’t really matter what Vriska did to her body since they’re aren’t alive anymore.

Not necessarily! Roxy is called the leader of the alpha kids but Jane was actually the first one to enter. So it’s up to the team.

A session with multiple players is a collaborative effort to create the players via ectobiology, complete the quest of each planet, defeat/deal with each denizen, breed the Genesis Frog, defeat the Black King before the Reckoning finishes, and finally perform the Ultimate Alchemy. 

A session with one player becomes a quest not to breed a new universe, but to gain unconditional immortality and endless power by completing a series of excruciatingly difficult tasks. 

A dead session is a session like Caliborn’s where only one person enters the game. Once Caliborn had finished his quest it wasn’t explained what he was supposed to do afterwards. In his masterpiece he says he’s hanging out with Lil Seb and Gamzee “as always” so it could be that players of a dead session just stay in the Medium.

Well she already had her mom in Rose back so she probably didn’t see the need.

They’re robots

Created a Giant Black Hole that presumably swalled everything in the Furthest Ring