I would go ahead and add the epsom salts. It can't hurt. A lot of pro growers do it. This is the thing with using coco nutes in soil, you might not get the normal problems other people get. Sulfur deficiency is pretty unusual but it could be happening. I haven't needed the salts but I have been supplementing with Cal-mag which has a lot of minerals and such that the Monkey Juice might lack. I have seen epsom salts at the hydro store but I'm sure you could get them at any nursery/gardening shop, maybe even the grocery store.
And yes, the super yellow light in my pics makes it hard to see the yellow on the plant. I have another plant just starting to flower, I can get some better pics in three hours when the lights come back on. It looks almost exactly like yours.