Stormers I don't really know what to say.
You really do need to get that legal advice. They will be able to tell you what you can do from here and if it is too late to report the threats and abuse.
Lots of people here have said things that I fully agree with and gapants has it spot on.
If nothing else you need to remember that in normal relationships and normal break-ups nobody gets their hair pulled, nobody gets strangled, nobody issues heat-of-the-moment death threats and nobody spends time working out a detailed plan of how to murder their ex and her new family which includes a detailed idea of how to strangle a two year old child.
Good fathers do not let their children witness them mentally and physically abusing that child's mother. Good fathers don't tell their children about their plans to murder their mother, step-father and sister and their plans to subsequently commit suicide afterwards.
No threats are acceptable, not heat of the moment ones and not ones that are based on speculation of what might happen in the future.
The pressure he has put on your son is unbelievable. "If anything happens to you I will kill your mother and everyone else before killing myself."
No normal person, no good father, says that to a child.
In your OP you said this: "I don't know, maybe it's not a big deal, but I try to be civil all the time for the DS's sake and every month or so something comes up that makes me question his sanity.
I mean, 'Strangle DD'???WTF? How does that even cross your lips?"
So every month you are questioning his sanity and you are shocked to the core by his threat to strangle your daughter (and rightly so, it is shocking to anyone who isn't barking mad!)
It's not too late to do something to protect your son and your daughter.
As someone else said, he's proved he's capable of violence and he has clearly spent some time brooding on how to kill you and your family if he feels you deserve it.
Just because it has been years since you broke up, it doesn't mean he has given up his 'claim' on you or his 'right' to possess and control you in some way. Or his 'right' to punish you if he feels you deserve it. It's not just you, he's involved your whole family in this, including a two year old girl.
Don't wait too long on that legal advice, and don't worry that you won't be believed, are over-reacting, or have left it too late.
He's a contol freak and a manipulator who seems to be convinced that he can successfully lie to you about what went on.
And if you just let this go then you are letting him. But you don't have to, and other people outside the situation can see him for what he is. As you say, you've found yourself in that situation now, where you feel like the only one who couldn't see what was happing. But now you can see, you have to act and get your son and the rest of your family away from him. He's not safe, even if the worst thing that happens is your son growing up to think this is normal, to abuse and control and threaten and lie and turn dispicable threats into a joke. It's not normal, but by ignoring it you will be sending the message that it is.