I was just worried about the magnesium deficiency that showed up earlier in the grow but I got you. Definitely don't want it to take up more nitrogen.
Honestly didn't look like magnesium deficiency to me. CalMag isn't an element it's a combination of calcium, magnesium, and usually nitrogen. All elements work in conjunction and the plant will take up what it needs from the feed assuming a balanced feed. (One element not locking out another, possibly what you are experiencing from excess nitrogen)
You know... I grow in soil and was serious in my advice.
I’ve had good experience using Roots Organics dry amendments in my 275 gal water totes and letting them circulate with a submersible pump overnight. Their dry amendments are super fine and nearly create a solution and you can apply it similarly as liquid nutes but they are cheaper and keep up microbial activity in your soil as opposed to using salts.
the Roots cal mag is formulated with zero N but does have sulphur. If watered to heavy runoff with molasses and left to dry out fairly thoroughly and then repeated being as its soil and should retain moisture for awhile, so you should see some results after that amount of time. After that you hopefully will have a way better idea of if cal mag is appropriate or not. I had a similar issue with a raised bed of white runtz last year and this snapped them back. Every strain has different needs and there are so many variables, considering, so trial and error is a great teacher in my opinion.
You realize molasses does nothing for plants right? It's there to feed bacteria only. As for the opts plant it's good looking at old growth is pointless cut it off and focus on forward growth.The molasses is benign (and cheap) and will aid in freeing up lockout if there is any, in addition to adding an easy dose of potassium which it looks like it could use. That would also make the N they’re being fed more available without dumping in unnecessary nutes that could just add to the problem. A little cal/mag boost before transitioning into your flower nute regimen has never done me wrong. Of course I have little information to base this on. I have no idea if this is live soil, coco or peat base. I’ve done this for 26 years and my thoughts are just a thin slice off of a quick glance at a bad picture. 
I was gonna do defoliation on all the lower leaves but then the yellowing will go to the next set of leaves. I think I'm do it regardless. We'll see what happens. Do you guys leave the plant tied down all the way through flower ?
It shouldn't move up. As for tying stuff down I don't I just top and run with it. You can see how I do my plants in my post " currently in my world"
I leave mine tied down as long as necessary. Towards the end of flower they've already stayed where they will finish and tie downs can be removed.