Freedom And Security

Death Monarch loves Katarina. This affection is known to all. But, his affection only extended to Katarina. 

As for Boris, they have mutual dislike of each other

Of course, Death Monarch would not kill Boris because that would put his relationship with Katarina in an irreconcilable difference.

However, if Boris did something that went against Death Monarch wishes, Mister Moscow do believe that Death Monarch would impose restriction on him 

In Japan, there might be a way for Yomi to once again establish their presence. 

The Kazoku system is rumoured to be established again

With the secession of Japan from the World Government, there is many powerful people in Japan that had flee the island nation and defected to the World Government

Yes, Japan has Oreki and Hikigaya. And yes, with those two, the World Government would find it hard to win a war against Japan. 

Because the World Government only have one Divine Comprehension leveler.