My pleasure, Archus!
If I were in your shoes I would not do Epsom baths during water fast. If absolutely necessary, I would do the first 24 hrs, then after day 21, half/half sea-salt/Epsom + a tablet of calcium carbonate and a potassium salt (no-salt or potassium iodide) in your bath water, ... since Epsom contains only one mineral and it may be potentially dangerous giving your body only one mineral, since a magnesium deficiency is not confirmed, according to your posts.
I am thinking that up to 21 days of a fast, you don't really need to be very concerned about minerals, except if you have some serious signs, such as palpitations, unremitting cramps, shortness of breath. For muscle/joint pains heating pads would be a better option in my opinion.
DL