While it could be magnesium-deficiency, it could alternatively be Tomato Chlorosis virus, or something else. Epsom salt is water soluble and can be used as a foliar spray without worry of what it will do to the nutrient-availability in the soil.
Oh, it looks like you figured it out! In my experience, Epsom salt in the soil has never caused any problems (unless you count my tomatoes acting like mild laxatives), but in theory, it's possible, especially if you get too much magnesium and not enough calcium. I'm not sure that Epsom salt would persist in the soil for long, though, but it's possible.
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