I have never had an OD of Epsom Salts...just start slowly with small amounts to test. Another way to get the Epsom salts to the plant is a light foliar spray of water and Epsom salts. Spray plants in the morning or evening when the lghts are off so the water doesn't give your plants a sunburn.
I agree with Phil. You need to get some mag to that plant. Water with Epsom Salt and you can foliar spray it with a diluted Epsom salt spray, within a few days you will see the color come back. It's common to see people forgetting to use some veg nutes along with the flower nutes.
My opinion on topping is that you need to have the room and some extra time for it. Actually my new mystic plants I just put into flowering will not be topped because I want to grow Christmas trees for my christmas harvest. I just want to produce a mega bud Christmas tree.
To make the topping...
Just the stem with an exacto on a 45 degree angle you want the root hormone to get into the stem so it stimulates roots. I have even cut small lines on the stem to give the hormone a place to take hold.
Cut stem.
Dip in hormones
Place in a peat pellet
Keep moist
Cloning is easy
Was grown from seed and vegged for about 1.5 months to get some decent clones before flowering. From dirt to the tallest tip its 66cm or 26 inches tall and is in a 5 gal pot. This plant normally grows real straight stems and needs some reinforcement because the buds can get heavy. I kept it...
I have a BM plant that is about 45 days into flowering. I'll take some pics of her today and post. Slow bud development over the first few weeks of flower but over the last 2 weeks she has packed on the buds. I can answer most questions about this strain.