sensi...foliar feeding/misting the leaves with epsoms is ..'ok'... untill you have corrected the defficiency in the soil. "Purple stems" ar not a sign of MG deffisiency. It is usually a genetic trait, but 'can' be from "P" defficiency. MG problems usually show up as "interveinal chlorosis" or fadeing/yellowing between the veins of the leaves.
rollingstoned mix sounds sufficient, but I can't agree with "every other day or 2". Once per week should be more than sufficient for feeding, and no more than 3 times per week watering. MJ likes/needs to dry out between waterings.
To simply avoid the MG deff. from the beginng, add 1 isp of horticultural lime per gallon of medium, "prior" to planting.