I'm pretty sure your runoff numbers are off if using the ph test drops. They have a color-coded chart, right? Tinting of the runoff water (and likely the nutrients themselves) skews test results.

If that's the largest pot you've got you might be able to get away with slipping-out the semi-dry rootball and adding potting soil to the bottom of the pot, and the sides once it's back in the pot. But you have to be sure to allow enough room at the top for watering. If still in veg you might be able to slice the roots and re-pot, but I wouldn't advise it unless you have a back-up plan. Tiz an advanced technique. Sort of.
Once in flower, you shouldn't dick with the roots too much.

What makes you think you have a magnesium def. and have you read the thread about epsom salts not being a miracle cure...?