Oh Megan I’m so sorry you and your kids are going through this. It does sound like he is reacting to some kind of trauma with his dad. Which is horrible. My marriage was abusive and I am still furious at the lasting damage he has done to the kids. Mine are grown now but still struggling with the repercussions. I pray you are able to find enough proof to get these visits stopped as it seems to be driving the behavior.
Are your kids still in regular counseling? If not I would push for that for both of them.
I would also make sure your kids always know you are a safe place to come to talk about ANYTHING that may be going on. It’s dicey trying to push TOO hard for specifics of what’s happening with dad - with the courts, you don’t want to be accused of putting words or thoughts into their heads or you may be accused of ‘alienation’. But you can ask them How things went and if they seem upset or if the youngest is especially difficult after a visit ask if something upset them. And you can keep them in counseling - a professional knows how to ask in the right way and will be an objective third party for any future court action.
I disagree that you can’t work on yourself until your child is better. In fact, I would argue that taking care of yourself, including battling your own demons, is an essential first step. You know why airlines always tell you ‘put the oxygen mask on yourself before helping your child?’ That’s because if you are strong, you can always help your child, but if you go under, your child cannot help you - and will have no one left to help him. You have to keep yourself strong. If you are not in counseling yourself, please look into some options. There are low cost/sliding scale clinics. And even online options now, though I think sometimes face to face is better. But self care is not selfish, and it’s not optional. Your children need you to be strong, calm and stable. Your stability will help your youngest more than anything else right now.
Keep in mind ODD is a description, nothing more. It does not mean your child is fundamentally broken or cannot have a better future. It is simply a description to apply to his outward behaviors right now. None of us can read the future. With the timing of when this started, it suggests strongly to me that this may be triggered behavior, which means that getting to the bottom of the trigger is just as I,portent as medicine and other treatments. But of course I could be wrong on that. There are so many variables with these things, genetic and environmental, that it really takes a professional to sort it all out - or several. I pray you and the kids get the help you need.
In the meantime, keep posting here. Vent, cry, get it off your chest, and hear from others walking this path.
Big hugs.