I am sorry it has taken me a while to respond to your comment. I sincerely hope you have found some comfort and support after the experience you write about, ending up 1000 miles from your home and ex, in a motel.
I have heard of and worked with multiple individuals and couples who have had your same experience, and have also found their Narc. Ex to be much easier to get along with when they are altered, whether it be from recreational drugs and alcohol, or from them using prescription drugs. I do believe that what generally makes someone feel more stable and happy, does tend to curb their behavior, but never ends up completely eradicating their tendencies in relationship, or the faulty and traumatized self concept and logic that guides being abusive.
There are also several people who have Narcissistic wounding who are also Bipolar. Both can exist together. Sometimes those with volatile personality disorders can be co-diagnosed with bipolar disorder and their personality disorder. The key thing here is to determine whether the mood cycling creates the dysfunction, or whether it is a structure of personality, lack of empathy, and behavior, paired with volatile emotions, that cause the dysfunction.
I am sending a very large hug to you, from afar, and hope you are doing well, if not at least, ok.
If you need any resources for support, just let me know.