What was the humidity like before you noticed signs of going into shed? I've found that if I keep a constant ~50% humidity and just bump up slightly when they are going into shed, I have far better results than just thinking about humidity when they're going into or in shed.
Saggy skin is perfectly normal when they're going into shed - it's actually one of the first things I notice about my animals before they start showing some other signs.
Nope - you read correctly. No need to soak.
Possibly - they're not exactly the Rolex of gauges. Here's the kind I have in my snake room: http://www.amazon.com/Quality-Import...rds=hygrometer.
Without knowing for sure, my thought would be to focus on humidity/hydration outside of the time that your little girl is actually in shed. Then, if you notice that she is still having issues, you can really crank it up when she's going into shed. I have a couple of big girls that I have to make sure to "baby" more than the others when they're going into shed otherwise I end up doing more work with them after the fact.
Hope this helps!
Best regards,
Eric