Well here is what I can tell you and as I said someone who has more experience in crack and cocaine will be along.
As far as what you are dealing with - you need to read, read, read about addiction and understand the totality of the disease. You have some huge misconceptions. "Beautiful girl from an upper class family....a real contradiction".
Addiction and alcoholism isn't choosy or picky - its face isn't a homeless person, or an unattractive person, or an uneducated person, or a person from a "bad" family. Its face is all people, from all walks of life, with all levels of education, from all types of families.
You will find a higher level of sex addiction and risky sexual behavior among crack/cocaine abusers.
I know you are struggling with this - but here is what you have written - you can't trust her, she can't keep a job, she disappears for the night, you think she has been unfaithful, and you know she is using, she is a liar and a thief.
The only positive thing I see written is that your son loves her. Lets think about that, she is an addict - and she is around your 5 year old son. ****DANGER***. Seriously, I don't care how much he loves her she is a crack addict. You need to remove your child as far away from this toxic person as you can possibly get him.
I know you are worn out, stay with her - and you will know exhaustion you never thought possible.
Stay on here, post often - Nar anon would be a great place for you to go to get help for yourself and help you understand what you are up against.
The three C's of addiction/alcoholism are:
You didn't cause it
You can't cure it
You can't control it.