As far as my mind can see it. If your done your done. Then your attorney would be correct. But if you ARE NOT DONE, then seperation would be the BEST option. You get a break from the constant being with her, she gets her chance to do what she says, she gets her chance for action. She gets the chance to be totally responsible for her actions. Her. Not you. Her.
That is an interesting idea and may do some good. But FIB does not have to stop the process for a separation to happen: she is free to move out at any time. A legal separation is not needed to do what you suggest. They can separate until the trial date. That will be a long time; I don't know about NY but I was separated 16 months from the time we filed until the time we settled out of court (and we still did not have a court date yet).
FIB cannot move out b/c custody is a real issue given her history with the kids. But she can move anytime. Her moving would also be more cost effective and would really give her a taste of life alone. In fact, this may be a way for her to demonstrate that she is serious about saving the M, not just saying she does not want to D. Time for her to do some of the work.
But I imagine if FIB suggests this she will say no.
And for the record my 16 months did nothing to save my M. I consider it wasted effort. But my situation is my situation, not FIB's.