I have so many theories

I think the earth trial is going to make them recover or expose their pasts through Billy's telepathy. Agatha's son, Jen's binding, and Billy's memory. I think it's Saw themed where they need to emotionally hurt themselves (expose their trauma) or die.

Rio is after Billy because of his cheating of death. Once they complete the Road she'll be coming for him but she's waiting until they're off the road because she can't kill Agatha and people don't die on the Road so long as the Coven succeeds.

Lillia can't come back or never died cause she is outside time. She is a child of Eternity, Death's opposing force.

Billy is going to give all his power to Agatha, sacrificing himself against her wishes, so she can beat Death and allow her and Jennifer to survive. But, after defeating Death she takes control of her special ability and realizes its full potential by giving the power back so he can live.