I also have done quite a bit of photo re-touching and it seems that you are on the right track by using Levels etc. I have to say that quite a few of these are, IMHO, way beyond repair and I am sure that the professional re-touching houses would stay clear of many of them. I always found that the hardest part was where the damage was on the face particularly through the eye. No matter how careful I was, when I had finished it never looked like the same person. First of all change them into Greyscale and go for it. Good luck