I am not sure how you define an "active EA". Your W wants the fantasy, and she is obviously chasing it down the wrong avenue. An EA really messes with the head. And don't think that b/c the OM is long distant that it's safe. It isn't. Lots of damage can be done via Internet.
As far as you setting a boundary about her contacting the OM, what would be the consequences if she chose not to honor it? I mean, the two of you are already separating, so what else could you do about her contacting him? See, a boundary is completely worthless if you don't have something as a consequence for her dishonoring it. Being separated limits what you can use as effective boundaries. Be sure you know the consequence, before opening your mouth to sound off some boundary.
Have the two of you drawn up any type of a financial agreement, child visitation schedule, etc.?
A lot of women want to live the single life and keep a foot in the M, too. They want the best of both worlds. We call it cake eating. They usually want all the advantages they had in the M, without the disadvantages. You know, like enjoying family times together. Doing things "as a family".......and of course, it's always "for the sake of the kids". No, it's for the sake of the WW! Everything is about her. She will still want to rule the roost, and partake in family events, maybe even have her time to cry on your shoulder when things don't go like she thought they would, or she'll expect you to run over ever times she beckons...........you know, kind of like she is still playing the role of your W, but she's not really.
I hope you get a plan of action in mind, and not fall for her games. She is going to try to play you, so be on guard.
I suggest you start with a massive GAL.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!