I think domi reversi will fail after Roger and Garp combo him. This might turn him back into a human, but after that he'll protect the ship his family gets on and all the holy knights will stab him during it.
That covers : Roger and Garp being allies. Garling champion of Godvalley.
I think Rocks will resist Domi Reversi for a bit, up until Eris dies, then he'll fully crash out.
We haven't seen Rocks meet any of his crew, yet, and Xebec has been heavily tied to Newgate, obviously: I think Davy D. Xebec will give the same order Newgate did in Marineford: "Get the hell out of here, men!"
As Rocks fights Kaido, Linlin, Newgate, we'll see flashbacks of him meeting them, IMO. Oda's gonna hit us right in the heart.
Right before Xebec is fully corrupted, he'll order his Crew to flee the island.
Roger and Garp will fight Rocks once he fully loses himself, until Harald shows up, then Harald will put him down, and Roger and Garp will knock Imu/Saturn back to Marijoa, like Mars in Egghead with Luffy, Bonney, Sanji, and Franky.
After Rocks is dead, we'll see God Valley fallout, then return to Loki and see how Rocks' death effects him, causes a rift between him and Harald, and leads to Harald's Domi Reversi. Need to get Harald tearing his horns out, too, I think that's due to killing Domi Rocks. Harald will associate his horns with Demonology.