Nutrient deficiency? If yes, what's the best course of action to salvage the situation?

Nitrogen my friend the yellowing of leaves usually means shes in need of more feed looking by the size of her im not suprised just up her feed and this should sort its self out in the time it takes the plant to uptake it probably about a week or two in soil :) the brown spots is again a underfeed and that one is cal mag rememeber that the elements NPK and cal mag are mobile nutrients within the plant they are in constant use no matter which stage of growth you are in anyway you should be fine them leaves will be consumed and fall off will be replenished by fresh fans dont worry yoooull be fine brother :)