Might methotrexate help w/dysautonomia symptoms caused by autoimmune disorder like lupus when standard tx (floninef & salt)don't' work but prednisone does?

May not be best. Dysautonomia due to lupus could be due to neurological involvement such as autonomic neuropathy, or due to adrenal suppression as a consequence of prolonged steroid treatment. In the first case Methotrexate might not be a strong enough immunosuppressor; its role in lupus is mainly limited to treating arthritis. In the second case, the treatment would be supplemental steroids such as florinef (fludrocortisone).
I'm no expert but. I don't think it will. The question is what the symptoms are and whether or not they are related to lupus. Sometimes Phentermine may help or compression stockings if it's low blood pressure. Maybe the sx's are in part due to anxiety and insomnia, chronic illness has 2x the rate of mood symptoms. It's a difficult topic thus you have not gotten a reply. Easier meds in a targeted way may be a...
It is not known. The setting you describe seems rational. Prednisone is anti-inflammatory and Florinef (fludrocortisone) is not used as an anti-inflammatory . The mechanism of the dysautonomia can be related to inflammation of the neurologic system and this Methotrexate may work. As you know therapy for dysautonomia is not clear but you are fortunate that yours responds to prednisone. Sometimes it does not.