Yes Roxy does canonically have a crush on Dirk. She states that she loves him on this page. 

The Condesce destroyed the moon so Dirk and Roxy could ascend to godtier.

Here you go.

Gamzee clubbed Nepeta to death with his Deuce Clubs. You can see her blood on his club during this flash.

The Vast Glub was a psychic shockwave released by Feferi’s lusus that killed every troll in the galaxy except the trolls with fuchsia blood who are Feferi and the Condesce. The Vast Glub was released right before Feferi’s lusus died. 

Yes Jade knew his dream self would be revived. 

Sacrificial Slabs are located in the center of the moons of Derse and Prospit so just go there once you enter the Medium. 

Rose and Kanaya are still together in this timeline judging from the way their interactions are. Kanaya was holding Sollux’s arm because he was blind, they’re just friends. 

The beta trolls are not clones of the alpha trolls. Karkat created 24 trolls during his ectobiology. Trolls are usually conceived when the Mother Grub combines adult troll reproductive liquids and then lays many eggs all at once. Karkat may have had to do a similar process but with paradox slime for 24 people.

Karkat created 24 paradox clones consisting of his group and Meenah’s group. They were then sent back through the portals, twelve would become ancestors and twelve would become Karkat and his friends.