Amber and Johnny have both had some much therapy but neither of them have really beaten the demons of their childhood trauma.
I really feel for Whitney. Trying to be the peacemaker and internalizing your pain takes a real toll and she has felt the wrath of Amber many times. She finds a bit of a kindred spirit in Johnny. Someone who gets her pain and understands what she still deals with and who also numbs their feelings with drugs / alcohol, then Amber ruins that too. Jennifer Howell had it right that at some point Whitney needs to leave Amber to her own devices and live life for herself but she still seems far too enmeshed in being the rescuer and being needed by Amber in some way and needing Amber in some way to move on.
I think Amber does have some signs of PTS (and BPD) She grew up with two parents who were both addicts and a father who got violent when intoxicated. I think that is why she freaks out so much when she thinks Johnny might drink again or use again and that is why she associates it with violence - even if the violence isn't happening now. She is projecting her childhood onto him and feels justified in getting physical and mean - probably things she saw or even did in her household as survival and defensive behaviours and she has normalized them. It is just out of context with Johnny but he has enough similar characteristics (substance use) and behaviours (anger) that she struggles to separate them. Her fear of abandonment is massive and her 'need' as she describes it to fight - shows she doesn't really have other ways to problem solve or resolve conflicts. She resorts to how conflict was handled as a child and still uses the same behaviours.
You would just think that with over a decade of therapy and counselling - they would have made more progress than they seem to have made. I think Johnny probably had done a lot of work but the dynamic with Amber put him right back into being a child being abused by his mother and he unravelled a bit.
They could both use a complete break from the public eye and a year of simple living with anonymity somewhere (not the same place!) to get regrounded and move past this, whatever the outcome.
That is my pontificating for the hour, now to go do some work before the trail starts again!