[GabrieliosP]

Spoiler for answers:

Its unknown what happens to Miyo after the ending of all arcs, but I personally think that Nomura got rid of her after that.

And it's plain obvious that Miyo made the Yamainu kidnap Rika's mother and dad. She killed Rika's dad and dissected Rika's mother alive like she did to the manager of the dam constructing team.

While that theory about Rika's "Mi" and Hanyu's "Au" is good, they've talked like this even away from the presence of each other. But we can also assume that they've done it so many times that it has become a kind of verbal tic.