The Beta kids didn’t decide the time period the Alpha kids were sent back, it was Skaia who decided what portals to send them through. 

They did have a reckoning.

Typheus knew another John from the pre retcon time line would bring his Lowas with him into the post retcon timeline. Also, Jade spending three years by herself without John or Davesprite was the price Typheus warned would happen. Because John was able to edit the timeline himself his friends had to feel the same loss he did. 

Because they are the product of two sprites merging together. 

Yes, she was killed by Gamzee.

They all died long before she entered her session.

Here.

Yes, he is they just look and act very similar.

The Vast Glub is a shock wave of psychic energy that was released by Feferi’s lusus before she died. It killed the entire troll race except for the Condesce and the eleven troll heroes who made it into the Medium before it was released. Karkat didn’t get Sollux into the Medium in time and he died. Feferi revived him afterwards. 

Yes, they’re in a relationship.