You were not at fault.
It is vanishingly unlikely that anything you did or didn't do even caused any major dissatisfaction and highly likely that even if you hadn't been bereaved and using crutches that he himself used, this would still have happened.
One of the hardest battles to win for someone in your situation is to get to the realisation that you could not control this.
It's terrifying.
We all like to think that we can control our own outcomes, but giving someone complete trust means accepting that this person has the ability to change our outcome. This is because none of is is able to control someone else's behaviour and if people choose to behave badly, there's not a thing we can do to prevent it.
It's interesting that you say he cut people out of his life for petty reasons. This gives further clues about self-aggrandisement and a sense of moral superiority.
I would say No to Christmas Day. Be firm about that to him, even if you privately make a deal with yourself that you can retract that if things change over the next few months.
I'm going to outline a failsafe strategy for dealing with this and I hope it will help.
It's understandable that right now you want him back. But that might change, so you need to have a plan that covers both eventualities.
This is a win-win fake-it-to-make-it plan.
Publically and to him, you tell him the relationship is over and that all you are concerned with now is a co-parenting, business and financial agreement. You stop having any conversations about 'us' and if he tries to, you stop him and revert to business discussions.
You insist he sees DD outside of the house and you give every appearance of getting on with your life as a single woman and parent.
Over a long enough period of time, this will be less about faking it and more congruent with how you're actually feeling inside. It's at this point or just before that when Romantic Idiots (RI) usually want to come back, but often by that point the new life for the discarded partner has become far more appealing than the old. So she says 'No thanks'.
Some people say 'yes' if the affair ends before the process of true detachment has taken place, but that only works if the RI makes very fundamental changes to himself. Some people say 'yes' and regret giving up that independence and better life that was so hard-earned, especially when the RI has learnt nothing.