Why does Scott address it “to the public”? Try to define who he means by “the public”.

Scott clearly addresses this message to "the public", probably because he does not want his diary to published. The diary shows the struggles of the expedition and the last thoughts of the men. The "Message to the public" is, in contrast to the diary, very formal and meant for the general public.

In the message Scott praise the courage of his men and gives a lot of external factors to why the expedition went wrong. Though Scott addresses his message to "the public", he probably does not expect every average person to read it. The purpose of the message is to explain the failure of the expedition to the reputable explorers and maintain the good reputation of the Englishmen.

What does Scott say about the English charter?

Scott said that they have taken risks and show that Englishmen can endure hardships, and help one another, and meet dead with as great a fortitude as ever in the past.

What is Scott’s main concern at the very end?

Scott's main concern is that they will be forgotten as men who failed big time.

From the text

How does he account for the failure of the expedition?

They did not expect Evans Edgar to fail, because he was one of the strongest in the group.
They don't think that he will succeed, because of the conditions (the weather etc.).

He did not blame his men for the failed expedition, but the conditions they were exposed to.

Compare the style of the diary to the style of the letter to the public. In what ways to they differ?

The logbook is very different, in some way, to the letter. The logbook is more honest, because it is not meant for others to read. On the other hand, the letter is meant for others to read. It is more official in it's language.