I haven't seen the Disappeared episode yet so I'm not sure all that was shown about her case. But I have followed it since the beginning.
I joined one of the fb groups and remember her sister was quite off putting, not wanting people to search or even get involved. Maybe she just thought she had gone away, but it never looked that way from my perspective or others asking questions etc.
I believe the sister informed Amber about the cheating, and that was when Amber stopped posting and took down her pictures with Katelin on fb.
On the show did it say that there was cash in the wallet? There was somewhere around $20 in there. If someone has nothing...and they are running away...you can bet they can use $20 so it would make absolutely no sense to throw it away.