Ouch. I'm sorry you had a rough go of it...
Given a choice, I would rather have had my parents be able to at least show up in the same place at the same time at a school function or the birth of my kids, without having to act as a travel agent for them so they wouldn't run into one another.
When my kids were born my father had dropped back into the picture (I actually had nightmares about that during my first pregnancy and he dropped back into the picture when I was 7 months pregnant!)...
When both were born I had to deal with my mother saying stuff like, "Well I won't come and visit you if there's a chance that Michael will be there" (visit in the hospital).
I managed to "schedule" them at different times when my daughter was born - I was only in the hospital 2 days and my paternal grandparents were here from England so I had to carefully make sure that nobody ran into anybody else.
When my son was born I had a section and was in hospital for 5 days and was on morphine for a few (complicated surgery)... when Mom started in on the "scheduling" crap again I had the drug-induced nerve to tell her that I wasn't a travel agent and if she was "afraid" to pass my father in the hallway, then don't bother coming. (OUCH!)... she came, he came and thankfully nobody ran into anybody else.
As for boundaries - we have them. As I said, XH and I don't hang out together although we do still work together in our jointly-owned business. I'm full time, he's very part time. I don't see his coming over to have Christmas dinner with us as being a "big happy family" - that's not what we're doing, at least I don't see it that way, I don't think my kids do - they aren't confused about where he and I stand with each other.
There's no "lying" going on. We're friends. There's no rule that says ex-spouses have to hate one another or make things difficult for one another or go out of our way to inconvenience the other. I'm not really going out of my way to invite him for dinner - I was going to make the dinner anyway - now he doesn't have to and he can spend time with his kids - and while he's over, I expect he'll be doing things with the kids - watching a movie or TV or playing games with them (they are teenagers) or whatever. I will likely ask him to give me a hand in the kitchen - just as any guest would likely offer to help.
Nothing deceitful or improper about it. This way we both get to spend Christmas with OUR kids and nobody's deprived.
IMO it's best for all concerned.