I know exactly what you mean. It does take a toll. I have 3 sons. I know how this impacts them. Your son is what- 8 years old? I think that's right. My babies were 12 when their dad moved out. They will be 20 tomorrow. The garbage of the D still impacts them. All you can do is do what's right for them. You did the right thing by signnng that paper for your son. I know I told you about my nephew who got the dear john letter in Iraq. He has custody of his son now, but his son - a 7 year old - who now has ulcurs. Divorce harms our kids. What matters is how we respond to them. All we can do is do what's right for the kids.
I know your STBX is a loon, just as you do, but I applaud you for doing the right thing by your son. Kudos to you.