The three nutrient deficiencies that typically cause interveinal chlorosis (which is still there on the older leaves) would be magnesium, iron or manganese. You've already tried magnesium (Epsom salts) which would usually affect the older leaves before the younger, and iron which is the other way around. That would leave manganese which would be harder to test for unless you can get some in a small quantity from a drug store or a garden centre that carries unusual items, or a hydroponics store.
There was some response to the iron, though, judging by the green spots.
Can you try planting it in the ground to see what happens? As I mentioned before, I know someone who had a daylily seedling that stubbornly stayed yellowish even into "adulthood" while all around it were normal. I'd be inclined to put it down to some problem within the plant that causes it to be unable to have normal green leaves. Whether that's a problem with chlorophyll production, maybe related to an inability to utilize certain nutrients, I have no idea.
