Do the Japanese Murder Hornets pose a existential threat to Honey bees in America?

Not particularly. 
There are still honeybees in Japan.  If we run out here, we can import some from Japan, where they coexist with the murder hornet.
You know that honeybees aren't native to the Americas, right?

No.  It's easy to outfit murder hornets with tiny tracking devices that they will carry back to their hive.  And once the location of a hive is known, it's easy to kill the entire hornet colony.

i would hope not