It’s certainly sounding like more than a mood disorder. Her thinking might be considered eccentric, but has she ever shown herself to actually break from external reality?
It is astounding that anyone puts up with her behavior. I would be very curious to get their perspectives. She may present a tad more normalcy in front of outsiders.
If I were forced at gun point to come up with an opinion, it would be that on some level, Mother-In-Law has a bit of awareness about being mentally ill. And, it scares the crap out of her. That’s why she lashes out so hard when the suggestion of treatment is made.
What is the story with her husband? Is he vertebrate enough to stand up to her? He could try to two card her; either you go to the doctor, or I’m out of here.
But, he has to really mean it.
Is her condition deteriorating? Who is providing the tramadol and Gabapentin? Both have very high abuse potential. And they do tend to react when taken together, albeit moderately.
What is her actual diagnosis to justify the tramadol and Gabapentin? Either of those meds alone could cause confusion and impairments in thinking and judgement.
Any effort to address her mental health issues has to begin with the meds she’s on. How old is your MIL? Do you know how long she’s been on those drugs?
Quite telling that she refused antidepressants, but willingly takes two high abuse risk drugs.
Unfortunately, unless she is a threat to herself or others, there is not much you can do. The onus is really on her husband to coax, cajole, or order her into treatment. Remember, even if there are really severe psychiatric symptoms, the decision whether to involuntarily admit someone to the hospital is based entirely on their dangerousness.
From what you’re describing, he probably should pull out all the stops to get her some help.