Could be a mag, really early to tell, i would have guessed maybe a cal, but cal mag would fix that
You should be using that from the beginning. I use it with every feeding
It doesn't look like you hit her with too much food, or at least no that bad. By far the worst case of overfeeding I have seen was when my moms bf got it in his head that twice as much fert. equals twice the speed of growth. Needless to say, they didn't even survive the first dose of that(mind you they were outside in planters, which the sun doesn't help at all with nute burn). Overall, the extra food you gave them shouldn't really affect them a lot, as long as you either go light on the fert for a couple feedings, then just bump it up a little once she starts to show that she needs something. The other route(a little more drastic) is to flush the soil, but it doesn't look like they are getting burnt at all, so you should have to do that(I always would hold that option until it had to be done, ie like the time i was stoned and put grow in instead of bloom)
It doesn't look like you hit her with too much food, or at least no that bad.
I think you are spot on , it looks like a Phos deficit to me too.