After seeing the state of his child, Bosse decided to make a kingdom so he'd live without want. Whether that was in service of his paternal affection, pity, or as a means to prepare his next vessel (and how much control of Bosse is ceded to the demon at the time) is up for debate.
For the Mirror to call Boji the most powerful in the world is suspect at best. The mechanism at play is obscured. Since it cannot be physical strength, I presume it means the general 'formidability' that anime often uses "strength" as a surrogate for, but the jump is too quick for me to buy it. That scene was necessary to inform the characters that Boji is still alive, but there's no tension in this reveal yet without Boji having a clear next step or goal. I'm surprised that wasn't touched on in this episode, instead focusing on the Bosse kingdom storylines.
Glad Bebin is still with us.
Daida's posessor calls himself Bosse, but I don't know enough about the character of Bosse to determine if what we have now is a full copy of his personality/soul, a compromised revenant, or some demon-infused variant, but it seems at least somewhat evil and territorial (ordered Domas to destroy the demon keep under the castle).
I'll welcome Hokuro into the big four. The more training arcs, the better!
Doubtful. He's planning to have her killed, after all.