Harley had scratches (mud fever) last summer. We had to treat it using a cream that contained penicillin.
I would definitely be taking preventative steps. My advice would be to trim any long hairs with scissors, wash the area with a tea tree oil based shampoo, dry thoroughly (like with a towel, and then let her stand around a while in a clean area like a stall, or on cross ties while you groom her all over). Then apply diaper cream thickly, focusing on the back of the pastern. Anything with a zinc oxide base will work, and did wonders for keeping Harley from getting worse or developing it again. As a bonus, the diaper cream will stay on a day or two, so every other day should do it. Maybe keep a brush just for removing caked mud (the hard plastic curry combs are good for that - about the only thing they're good for), and when you go back, just start by brushing off mud. If you can see there is still diaper cream underneath, just re-apply, you don't need to wash again. Over-washing isn't good either. If you can time it so you're there after she comes in from outside, you can clean her up for the rest of the day/night, and her skin can breathe for several hours, so going back out in the mud for a few hours a day won't hurt. It's more of a problem if the horse is in it 24/7 and/or never gets its legs cleaned.
And yes, all the vitamins and minerals are necessary. I know you're still waiting on your hay analysis, but the sooner you can start supplementing some basic vitamins, the better. You could already touch base with an equine nutritionist, if you have found one, to ask what you can safely add to her diet immediately (ie, without the results of hay analysis). Things like iron and selenium, you'll want to hold off on, but you may be able to add a few vitamins without any risk.