You have asked far more than one question so I have edited it down to the one quoted above. Enotes specifies that you may only ask one question each day, so make sure you remember that for next time!
When the bet is due to be finished, the watchmen rush in and tell the banker that the lawyer has been seen leaving the lodge leave and disappear. We are told the following:
The banker went at once with the servants to the lodge and made sure of the flight of his prisoner. To avoid arousing unnecessary talk, he took from the table the writing in which the millions were renounced and, when he got home, locked it up in the fireproof safe.
What is interesting about this behaviour is that it seems very different from the banker's actions just a paragraph before. The reading of the lawyer's letter makes him feel "so great a contempt for himself", so much so that his emotions and tears kept him from sleep. However here we see that he hides the letter because he is ashamed. Also, preserving the letter makes it even worse as it suggests that he wants to keep it for evidence, because he does not trust the lawyer's declaration to give up the money that he could have earned. This demonstrates that the banker, in sharp contrast to the lawyer, has not evolved beyond his greed and self-interest.
These characters and the plot were invented by the author Anton Chekhov. No doubt Chekhov wanted to finish the story with the reader feeling assured that the lawyer would not change his mind about forfeiting the money and come back to claim it. There is plenty of evidence in the lawyer's own handwriting that he has forfeited the bet and cannot claim the money, in spite of the fact that he actually spent the full fifteen years in confinement. The most important words in the lawyer's farewell letter, as far as the banker is concerned, are these:
"To prove to you in action how I despise all that you live by, I renounce the two million of which I once dreamed as of paradise and which now I despise. To deprive myself of the right to the money I shall go out from here five hours before the time fixed, and so break the compact ..."
We are in the banker's point of view throughout the story. Although we can sympathize with his feelings, we do not respect him. He is not a good man. He was actually planning to commit a murder and then try to blame it on one of his servants. The banker may feel ashamed of himself, but he is still a selfish businessman, and he wants to keep that letter as evidence that he cannot be compelled to honor the bet, either by the lawyer or by anyone else connected to him, such as a relative. As readers we have to stay in the banker's point of view because that was the way Chekhov decided to tell the story. But our real sympathies are with the man who became saintlike through suffering fifteen years of solitary confinement. The banker won the bet technically, but the lawyer won it morally. The banker's character deteriorated over the fifteen years, while the lawyer's character improved.