"how is it bad for me and my kids for them to keep as much of their current routine as possible? (like me coming to their house to make them dinner a few nights a week)."
All I can tell you is: (1) Such a plan would keep your lives entangled in unhealthy ways. (2) The fantasy that such a scenario would work is as frequent (and as frequently false) among people embarking on the D road as the fantasy that one's WAS would-never-have/could-never-hide an A among newbies. Both are fantasies about how we would like the world to work that are seldom true.
Imagine that right after the D, XW has OM move in. But, she still wants you to come over and cook for the kids. Would this be OK? If not, then it shows that your comfort level hinges on what she does in her personal life. It shows that your life would still be about her, about her A, about her dating. Not good for anyone. Her personal life should have no bearing on you or your parenting. If you try to parent in her space, then you pollute your parenting with unneeded entanglement with her personal life.
As for her dating, I'm glad that you seem to be loosening your grip on the significance of that. Your life for a long time now has been all about her R's with OM. Time to make her R's with OM irrelevant to your life. The only rational thing to do at this point is to assume that she is dating and will date and base your choices on that assumption. For, your best evidence points to the truth of that assumption.
