What is the ruling on saying "be at peace," "be well," or "be amiable," and does this fall under presuming to know God's will?

The questioner's statement is not an act of presuming upon God, but rather an expression of hope or optimism for the recipient of good. Presuming upon God is when a servant says: "By God, Allah will not forgive so-and-so," which is a judgment by a creature upon the Creator, and it involves ignorance of Allah and ill-manners towards Him. It is reported in a Hadith that a man said that, whereupon Allah Almighty said: "Who is this that presumes upon Me that I will not forgive so-and-so? Indeed, I have forgiven so-and-so and rendered your deeds worthless."