Weaponized "Co-Parenting"

[Dionysius]

I don't know what to do.
My BPD ex wife and I divorced four years ago and eventually ended up co-parenting well together. A year ago we tried to reconcile, only after she'd received her diagnosis and I learned more about the condition. After a few months I decided I couldn't go back to dealing with the condition so I broke it off.

Ever since she'd been in a downward spiral (off work, hospitalized on suicide watch, etc).
She won't accept that I didn't/don't want to continue the relationship, so I get torrents of texts demanding explanations, telling me how awful I am as a person, and now increasingly making veiled threats about changing the shared custody arrangement we have with our son (12 yrs old).

She's increasingly projecting her own feelings on to him and using him as an intermediary and informant to monitor my behaviour. Particularly now that I'm in a new relationship the harassment, bullying, and threats have only escalated.

I don't know what to do and whether it's time to seek legal recourse. Though I don't know what the threshold for harassment is.

In the past I could "deal" with the harassment directed at me, but now she has dragged our son into the middle of it all. Her problems are his problems. Her feelings are his feelings, etc.

If I could I would go "no contact" but that's not possible in a co-parenting situation. Perhaps we should communicate through a mediator?
I'm really at my wits end.

[livednlearned]

It sounds like she has turned the dial up on parental alienation behaviors, like getting him to be an informant on your behavior.

Dr. Craig Childress refers to alienation as a form of emotional and psychological abuse  

How is your son doing? Does he know his mom has BPD? Do you have a good relationship with him? These are important questions because he may respond well to counseling, depending on the loyalty bind with his mom.

What is the current custody order and what changes is she proposing?

[Dionysius]

When my son is with me he is fine, although ex insists on how confused and messed up he is because of me.
He doesn't know about his mom's BPD, but he must sense something is up. I've always had great relationship with him but now she appears to be working to undermine that. Telling him that I've lied to him and prompting him to ask me questions.

We currently have shared custody 50/50. I assume her veiled threats mean she wants sole custody, but all the grounds she thinks she has for that are made up and imaginary. I'm trying to set up a meeting with my lawyer to see what my options are.