Hi CC420,
I just came across your journal. I used the MG organic on my grow and I'm having the same nute problem you're having. Leaves on my girl look identical to the lower leaves on your plant. Problems for me began about in the 4th/5th week of growth on all 5 of my plants.
I have not flushed as you intend to do. The roots on your plant are probably suffering now without the flush so it sounds like a good idea to do. I'll do likewise today hopefully.
Scott's soil is all that I can get around my parts. I used Scott's MG soil (time release, non-organic) on my only other grow and had similar problems. I hate monopolies. Fox Farm Ocean Forest sounds good but expensive to ship.
QS