Andrew, I think you're right not to be too excited because you are probably in for more rough times.
I remember from when my H broke up with OW1 that the sitch was nearly more difficult. First of all, the break ups didn't stick so I couldn't never really trust it. There was a period of to-ing and fro-ing which was painful. Secondly, when they did finally really break up, my H was so depressed that I wondered if he'd ever recover from losing the 'love of his life'. Obviously, your sitch may be completely different but I'd be amazed if it weren't even rockier for a bit longer.
In the big picture, it's looking hopeful, but be prepared that the next little while might test your DBing even more. I'd say just carry on with what you're doing. Stay put for now and keep showing your W that you're not trying to be controlling of her and that you have energy and time for the kids.
How lovely that D11 enjoys her times with you so much. It has to say a lot for you and I'm sure your W realises this. She must be your step-D which makes your good R with her even more noteworthy.
Good luck!
Me: 51 H: 52 T: 23 yrs M: 19 yrs S18, D16, S14 (special needs) PA: 2003/2004 Piecing: 2004 on Suspect H had EA: 8/2012-12/2012