Help with: Pre-flowering + First application of any fertilizer...

OK, so, I'm new here and I could really use some input.
I have an outdoor plant that has been on a strictly organic compost diet, adding select cuttings/compostables (nutrients) for every ailment I've come across, but I think I've reached a point where I, unfortunately, have to turn to these chemical fertilizers for nutrients.

My plant has just recently shown signs of being a girl, so since she's in the first stages/pre-flowering I'm not really sure of which fertilizer to use. I figured out, with enough research, that she has a Mg deficiency and so I've already added some epsom salt, but I also believe she is showing signs of a K deficiency...and of course she could use some more N, as the leaves are yellow.

Here's the problem:
I have two fertz to choose from (without buying more), either an 18-6-12 or a 10-35-15.

I know she could use some more P, going into the flowering stages, but the fertilizer with the higher P is pretty low in N & K, so could I mix these fertz?
If not (tending to caution), which should I use?
Couldn't I just add some lettuce/spinach/grass clippings/celery leaves to boost the N the way I have been?

Thank you for any input.