Legal Question in Family Law in Washington

Yes. Irretrievably broken is the standard.

Yes you can, because there is no rule that says you have to live apart. In today's housing market, this is becoming more and more common where parties need to split, are upside down on their mortgage, so that they would have to pay to walk away from the home, and because they have otherwise perfect credit, they work out a roommate situation instead. Not ideal, but in some folks' situation, it is all they can do.