HI there,
I'm sorry for what you have experienced. I have do not have the answers to what you have gone through with the epsom salts, only "ideas" but I have read warnings regarding their oral use before and personally I would never use them that way, but rather put them in my bath water instead.
I would imagine they have shunted toxins into worse areas perhaps and has made you very toxic, hence the headaches and nausea (which can be liver poisoning). I had something similar happen after being prescribed large dose vitamin D (50,000 IU). I was to take this for 5 days, then level off to taking one dose per week. It made me so ill, that I stopped it after the 5 days. It took me 2 and a half months to notice recovery. I have experienced headaches, nausea and other symptoms all that time! My system was unable to eliminate the excess vitamin D and I was poisoned by it. That is one explanation anyway. One doctor said that I wasn't metabolising properly, hence the build up.
I would suggest that you support your liver perhaps with a good liver herbal combination. Not "too" strong dosage, because even that can create too much "detox" symptoms (or retox). Just get some support. Drink good cleansing herbal teas and perhaps lemon juice added to warm water. Raw foods in fruits and vegetables are also good liver cleansing foods. But again, can also set off detox reactions, so just take it easy at first if you are not used to consuming raw foods much. Eliminating sugar/gluten/yeast from the diet can be very helpful in reducing overall toxic load and excess yeast levels, which often result from mercury poisoning or other immune conditions.
Do not fast with mercury poisoning. This has been warned against by more than one professional. If anything consuming good protein foods and good fats is recommended - eggs, meats, butter etc. These are protective and supportive apparently. You can help release some toxins and heavy metals by a good healthy diet, regular moderate exercise (walking/treadmill) and if you can cope, hot baths to encourage sweating also (saunas even better). But never go overboard. Only to aid the elimination of toxins, not overwhelm yourself. I cannot handle hot baths, but I can handle moderate walking.