Solution 1:

Dictionary.com referred to it in these meanings as a preposition:

  1. Up and on; upward so as to get or be on: He climbed upon his horse and rode off.
  2. In an elevated position on: There is a television antenna upon every house in the neighborhood.
  3. In or into complete or approximate contact with, as an attacker or an important or pressing occasion: The enemy was upon us and our soldiers had little time to escape. The Christmas holiday will soon be upon us and we have hardly begun to buy gifts. The time to take action is upon us.
  4. Immediately or very soon after: She went into mourning upon her husband's death.
  5. On the occasion of: She was joyful upon seeing her child take his first steps.
  6. On (in any of various senses, used as an equivalent of on with no added idea of ascent or elevation, and preferred in certain cases only for euphonic or metrical reasons): He swore upon his honor as a gentleman.

The last definition best represents the usage in your sentence and thus you may put either wait on or wait upon because both are acceptable; they mean the same thing. Wait upon seems to be seen in older literary works, whereas wait on seems to be used in more contemporary literary works. As referred in the earlier post wait on is more commonly seen than wait upon.

Solution 2:

I think you can say both "wait on" or "wait upon." However, I think "wait on" is more common.

In terms of hits on Google, for "wait on" there's 846 million results versus 250 odd million for "wait upon."

Here it says "to wait (up)on" so perhaps it is simply that over time, the "up" of "upon" was dropped. The example there where they use "upon" is in a medieval context.

Etymolonline says that "to wait upon" originated in early 1500s, giving the idiom time to change. Thus, I would conclude that either one is acceptable, but "on" is more common. That's all I could come up with. Hope this helps! :)

EDIT: Thesaurus.com also has it as a synonym with attend (to), be in the service of, do for, doctor, look after, mind, minister to, nurse, serve, take care of, tend, wait upon , watch, work for... etc.

Solution 3:

Yes, it is correct. The prepositions "on" and "upon" are often used interchangeably, though "upon" usually marks more old-fashioned use.

The phrase "to wait upon someone" means "to attend to / serve someone" (think of a modern waiter in a restaurant). There too it is old fashioned.

The full form "let us" instead of "let's" is also old fashioned. Since an usher is typically one who would attend to clients, patrons, or guests, that is probably what it means here.