Maybe related to this topic. Is there any gramatical structure for this sentence: "Could the lad not take a letter for me to the queen?" I have read it in a tale, where king requests a boy to do something for him. Is this an "order" or "plead"? (I know option 1 is more probable, but I hadn't seen a form like this.)
The construction you quote (with "could he not ...?") is definitely a polite request, not an order.
It does sound strange that a king would be so meek in his language. I thought at first it might be one of those frequent cases of a king going into his kingdom incognito, in which case he would probably act as if he were an ordinary subject, and not issue orders, but pleas.
However, looking into this, I wonder if the quote happens to be from the story "The Giant with the Three Golden Hairs", or "The Devil with the Three Golden Hairs". I read up (superficially) on a few versions of this story, and I found a few different alternative descriptions of the king's statement:
1) "Wikisource: "He is a fine fellow," said the king, "can you spare him to carry a letter to the queen?
2) From Professor Norberg, Duke Univ.: He hid his vexation, however, and presently said kindly, “I want to send a letter to the Queen, my wife; if that young man will take it to her I will give him two gold-pieces for his trouble.”
3) One-elevenbooks.com: He tasked the boy with taking a letter to the queen.
4) Howardism.org: Years later, the king, again disguised as a merchant, was wandering through the land, ... "I have an errand for the boy," said the King, "if he is man enough to undertake it."
If your sentence is indeed from this story, it seems that it is a polite request, and the king behaves in such an unassuming manner because: A. He might be disguised as a commoner and/or B. He is trying to appear nice. (The contents of the letter are anything but nice, but he wants to lay a trap.)
It is a slightly odd word order. Possibly it is an old fashioned word order. However, the meaning is the same as "Could not the lad take a letter to the queen for me?"
The sentence is structured as a question, but most likely it is a rhetorical question, implying "I think the lad can take a letter" and challenging the person he is talking to to contradict.
— I want to send this letter to the queen but I'm too busy. Can you do it?
— Sorry. I'm busy too.
— What about your son. He doesn't do any work. Could the lad not take the letter to the queen for me?
— Yes, I suppose he could.