I think it comes down to what you want in your heart.
If you want to save the marriage, then separation should be the next step, in my opinion. Since she is openly disrespecting you with regard to the OM, she needs to go, period. Let her see how life alone treats her. Then you can have your space as well.
Since she seems to be pushing the legal separation angle you have to ask yourself if you are done. I think if I were done, I'd say to heck with the legal separation, get it over with by getting divorced.
Only you can answer that question. But I sense you are not yet done. All we can do is offer support, and I hope it helps.
WAW Using God Me-43 W-40 M-14 S-11 S-9 D-7 EABomb 5/09 Separated 12/09