"Purple Secret" mentions the DNA tests performed to identify the remains found in Siberia, but does not provide any evidence that Alexandra suffered from porphyria; I think it does not even discuss the question whether her various health problems might have been caused by porphyria. And for good reasons, I think: Alexandra suffered from back problems, headaches, fatigue, anxiety at public functions, and an 'enlarged heart' (cardiomegaly?), but not from symptoms common in porphyria patients such as abdominal pains, vomiting, seizures, hallucinationsepression, etc.; the clinical picture of her health problems does not seem to fit that of porphyria.
She did have abdominal pains, paranoia, and depression. One does not have to have every symptom of porphyria in order to be diagnosed with it. And there are several forms of porphyria...one in particular being porphyria variegate which presents with a variety of symptoms. I do not think porphyria can be eliminated. Also the book devotes a whole chapter on the possibility of Alexandra having porphyria.