Learn English – What do we call this part of the room

The first is often called a mister - a device, such as a bottle, with a nozzle for spraying a mist of water, esp. on houseplants.

The second is often called a CD cakebox, defined by Google as...

1: A storage container for a round layer cake, with a surrounding cover that protects and preserves the cake.
2: A similarly shaped package for blank, recordable compact discs, with a central spindle on which discs are stacked.

As implied by that second definition, if OP specifically wants to refer to the "pole" in the middle, it's always called the spindle.

There isn't any specific English nomenclature for employees at a sushi restaurant. I have not been to any sushi restaurants that refer to their employees by the Japanese name of their position, but I'm sure they exist in the country somewhere.

Colloquially, managers are simply called that -- managers. There is a difference, however, between "the manager" and the actual owner of a restaurant. Managers are in charge of the restaurant while the owner is not in the office.

To answer your main question, I think Jason S described it adequately -- the person that takes reservations and seats guests is called the host or hostess, depending on gender. Those who serve food to the guests once they are seated can be called waiter/waitress, server, or steward/stewardess.