I expect all of your emotions have been turned inside out by this tumultuous, manipulative, enmeshed wrangling.
You have done well to step back and to step up so that you can see the messy toxic energy system that this other family dragged you in to.
Keep with the detached and calm approach - light and breezy but consistent boundaries.
This is what has worked for you and will continue to do so. I think having “the conversation” could well be counterproductive.
He will be disoriented and she will be on the ceiling so expect the tension to rise in your direction - but as you have dropped the rope it can’t reverberate from him to you and back again which is what feeds them.
Don’t walk on eggshells or let him sense any resentment from you from the last few months - because that will be you inadvertently giving energy to the system which will trigger him ricocheting between the houses.
This is a great step forward - but it still has a long way to play out yet and it is v fragile and volatile. The long game is required - hold your nerve. “The conversation” will have more impact when he is able to reflect in a calm and considered way - which is likely when he is many months out of this drama RS where he is hyper alert 24/7 and unable to properly emotionally process.
Be patient and bide your time. With abusive marriages / relationships the abused partner leaves and returns on average 7 times before the final time. Prepare for the worst and hope for the best.
Just make your home fun, light, busy, social, outward looking, collaborative, consistent, contained, boundaried - a direct contrast to the demanding and controlling emotional rollercoaster at his GF home. Pace yourself.