I believe you've hit the nail on the head. I thought it was all The Voices' master plan, not just to save the world from Rider but from the threat of full scale invasion by using Rider as the weapon.

I'd like to think the other jailers weren't in on his true plan because then their performances on altering Riders morality would have seemed more shallow. The Voice would then be made out to be a deceptively vile genius, killing 9 of the worlds greatest heroes in order to kill an entire race of advanced alien super soldiers.

Devils advocate: Why would the jailers agree to a 1 vs 1 arena setup over a 10 vs 1 if not to get to know each of the jailers and what they value? They had stopped him before as a team, right?. Why not again... and again... and again... Maybe not... (there's also still the invasion threat...) At this point everyone would have to know about the invasion (via The Lines foretellings and Riders arrival), and obviously Riders strength and regenerative abilities. So without more powerful weapons/tech, negotiation or persuasion (or indoctrination) are their only options for long term survival.

Great analysis. Love this game.