My RAW lost her job, blaming her lack of performance on stress from our "marital problems." Naturally, these stem primarily from her alcoholic behavior, but she hesitates to connect those dots. She also sometimes blames our problems on "my depression," which I am quick to point out, also stems from our lives becoming unmanageable due to her use of alcohol. It is amazing the degree to which otherwise intelligent people can find a way to rationalize their behavior, blaming it on others (who are themselves merely responding to the A's behavior).
In your shoes, it would be hard for me to resist taking her up on her offer of a separation. It might be painful if she subsequently self-destructs, but it sounds like you have minimal influence over the path she will choose - she may well self-destruct with you or without you.