if it crept up from bottom it needed nitrogen. if it started at the top, probably related to too much p or k.
your grow lights probably make it seem more yellow than it really is. Try to look at it under normal lights to guage that effect, but probably still going to be on the yellow side from what i see here.
senescance is inevitable. the plant will go down hill eventualyl while in bloom and die. it'll suck down the leaves during that process.
As long as the buds keep ripening, it'll finish.. i've seen far more damaged canopies than that get to harvest. I'd suggest not throwing everything but the kitchen sink at the problem. you want to try one thing, stick with it and see what happens unless absolute catastrophe ensues (some common sense here)...
things do not happen overnight often. changing gears several times means even if it did improe you'd have absolutely no idea which action you took had any effect. Get a good leaf chart, start referencing it until it is familiar. take notes of how symptoms progressed and what you did to stop it and what effect did your efforts have? try something different next time you encounter same set of variables. if enough time is available you can try other things, but make sure to give each enough time to actually make an impact before switching solutions.
running aound like a chicken with its head cut off won't get you far and just makes a mess. you may not always be able to correct what is going on this time, but you should at least learn something to improve probabilty of avoiding it next time.