Just wanted post a quick update...
At this point, I'm certain it's just shedding, thankfully. I haven't used the Betadine for a few days now because I wanted to see if anything was growing in the areas where I'd already removed the dead skin. And so far so good. No regrowth, no spreading, nothing. And she just released a hilarious looking head shed the other day in the tank, and I wish I had my phone nearby when it happened because it looked like a ghost in the shape of her head swimming around.
So it seems that these types of turtles just shed a TON, and some of the shedding just so happens to be horrifying in appearance. If anything changes, I'll make sure to post another update. Thanks again for helping me it, Steve
Here are few pics that I just took today. As you can see, there's a stark contrast between the pictures in the OP compared to this one.
Here you can see where some of the skin on his neck was partially removed: