That's pretty dark...
what is the difference between "adieu" and "au revoir"? Is one more final that the other?
Obviously, yes, "adieu" means "à Dieu" - "to God"... meaning more or less that you will next see the person again in paradise...
Then, Is this one used to express that one thinks to meet him/her not anymore?
And "au revoir" for quotidian talks?
Yes, unless it is a regional custom.
Is it inproper to say one or the other to someone you don't know ?
One or the other can be said to a child you don't know, there is no familiarity in there.
This is a very sad lesson.
"Goodbye my child" Traduit en Canadien: "See you later, kid!"