There is a reason why Hell no longer needs a warden, but that reason is never shared. With how vague God is and how everyone interprets what they expect from that vagueness it may be that Hell never needed a warden and everyone just thought it did, and Goid let them think that. This can also explain why nothing happened until Father Kinley alerted the demons.
Michael is shown as both a master manipulator and an idiot that has no idea what he's doing. He doesn't seem to have a grasp on self actualization as shown by his scar and loss of his wings. The loss and return of their wings is what lead Lucifer and Amenadiel to understand it. Yet at the same time Michael understands self actualization well enough to plant the idea that causes God to start losing control of his powers. He's not able to utilize it himself but also his plan to convince God to retire hinges on it.
There is no indication that anyone would receive God's abilities. Micheal expects to become all powerful. Lucifer expects to be able to change the rules of Hell. God took his power with him to the other universe. He never indicated a successor would get a new set of powers. The idea of a successor started with Lucifer and Amenadiel. It may be that God never intended to name one. The angels would have to work together to fill in for him since no individual would be able to.
I don't think God was manipulated. I think he planned to retire and, as Lucifer asked, everything else was part of that plan.
---
No locked doors, no windows barred. No more things to make my brain seem SKARD. Look at Mr. Technical over here >.> -BTB