Recognizing Deficiency. Need 2nd opinion

OK to start, I received this plant from a friend who planted it in a big pot of Jiffy Mix (meant for clones and seeds) and was asked to save it.

I'm trying to balance the nutrients so I dont have to transplant it 2 weeks into flowering. Originally, 1 week into flowering some of the big fan leaves got dark spots on them and the stems turned dark aswell (picture 1)

Within the next week, the plant grew several very small 3 pronged leaves that quickly turned yellow with dead brown spots (picture 2). I recognized this as a Phosphorus deficiency. First I flushed the soil, then added a 15-30-15 fert with the next watering.

Now about 4 days later, the plant has developed an obvious Mg deficiency which I am in the process of fixing with Epsom salt. I have noticed slowed growth and no signs of sexing day 15 into flowering. Notice in the white circle in Picture 3 how the new leaves are stretched out and curving at the end, while in red I've circled some of the persisting spots on the lower leaves. This still appears to be a (P) problem to me, I'm wondering if the right fertilizer could fix this or will I have to transplant this to a better medium?

-Doc Nomad