When God says "This is sure"

When God swears by himself, does this not mean that what is being predicted depends only on him? Yet some of these purposes, where God swears by himself, involve human decisions:
 
Genesis 22:16-18  "I swear by myself, declares the Lord, that because you have done this and have not withheld your son, your only son, I will surely bless you and make your descendants as numerous as the stars in the sky and as the sand on the seashore. Your descendants will take possession of the cities of their enemies, and through your offspring all nations on earth will be blessed, because you have obeyed me."
 
Many human decisions are involved here, yet this is quite certain, and thus God really knows the future, this is is not simply estimating. And similarly with Peter, how could Jesus predict that Peter would not remember, the first time the rooster crowed, and then remember the second time it crowed, if this is only character solidification? And when Peter says afterwards "You know all things," while being reminded of his denial, Jesus confirms Peter's statement quite directly, with another prediction about his future, again saying "truly, truly," implying this is as sure as the former prediction, and that this time, Peter will glorify God.
 
So saying "This is sure" means ... it's sure! We must not take the approach of discounting when God says something is certain:
 
Genesis 3:4 "You will not surely die," the serpent said to the woman.
 
"Since Jesus' words are preceded by the solemn 'I tell you the truth,' they are not to be taken lightly. To suggest that Jesus was mistaken in the statement he made in this verse but that the mistake was in a matter of such small consequence that it makes no difference is to fail to take seriously the solemnity of the introductory words." (Expositor's Bible Commentary, on Mark 13:30).
 
Matthew 5:18 Truly, until heaven and earth disappear, not the smallest letter, not the least stroke of a pen, will by any means disappear from the Law until everything is accomplished.
 
This seems to be more than just an underline on this sentence! Similarly, here:
 
Matthew 10:42 And if anyone gives even a cup of cold water to one of these little ones because he is my disciple, I tell you the truth, he will certainly not lose his reward.
 
Must we now say this means "This is a true emphasis?" This is important? Is that all that it means? Surely the meaning is more than just that:
 
Matthew 13:17 For I tell you the truth, many prophets and righteous men longed to see what you see but did not see it, and to hear what you hear but did not hear it.
 
Is that not really true?
 
Matthew 18:3 And he said: "I tell you the truth, unless you change and become like little children, you will never enter the kingdom of heaven."
 
Is it possible to enter heaven without this transformation? Why do we read "Truly," "Assuredly," "I tell you the truth," in the various translations, if what is meant is really only "This is important"? I think the translators are not all mistaken, in this...