My mother has been dealing with this for 15 years (she's 65) and she swears largely by alternative remedies. I honestly couldn't tell you what kind of supplement cocktail she's on now, but she researches this stuff all the time and is forever trying new things. There is no magic solution, unfortunately. I know that she swears by high-quality fish oil, vitamin D and b-vitamins and beyond that, it's something new all the time (iodine, whatever the key ingredient in Mucinex is, whey protein, vitamin C, probiotics etc.). She has done a yeast-free diet, gluten-free, dairy-free, etc. I think that all of her supplements and dietary changes do help her and make her functional (or at least don't make it progress) and it gives her hope and a sense of control that she can always try something else. She never stops searching for something better.
For exercise, she does swimming and walking, and very gentle yoga.
Probably the biggest difference for her is to know her limits and adhere to them. For example, she knows that she can't go to a function on a Saturday night and then do something on Sunday. More than one social event or work shift on a weekend would knock her on her butt for a few days. It's not worth the exhaustion and pain to push herself.
Many years ago, she was going through this at around the time she was going through menopause and the lack of sleep was just killing her. As much as she avoids conventional medicine (ironic because she's a nurse in a nursing home and dispenses meds all day), she did use a low-dose anto-depressant for a while, which helped with sleep and mood. I think she also used melatonin and St. John's Wort for these symptoms too.
Your MIL is lucky to have someone in the family try to understand her health issues who can validate and advocate for her. Sometimes just knowing that other people believe her and know that her pain and fatigue are real will be helpful. Hopefully you'll get clearer info from other sufferers!