Trimming

So, you can HOWEVER you must note that anytime something is cut from the plant, it will cause stress. And yes they wont grow any more leaves once flipped to flower. People do some extreme training with their plants. Sometimes trimming them down to just a few leaves when doing HST and LST but this is done in mid veg. At this stage if I were in your shoes and if the plants in flower, I would only remove fan leaves that are blocking primary bud sites. This will for sure increase yields. If this plant is NOT in flower and is in veg, I would not take any of the leaves off but address the Magnesium deficiencies first. Leaves showing deficiencies can help with telling you if you've fixed the problem or if its still progressing. I would look into calmag deficiencies, that will effect your flowering more than the leaves will. Good luck man!