Hi Jacquie, welcome to the board.
My Senegal, Maxwell, sounds like he is in similar condition to yours. He is about 8 years old and weighs 111g (this has been consistent for the last two years) and has bald patches under his wings and at the base of his throat. He also doesn't play much, but just mostly sits around.
When I first got him (he was a rescue), I was told the bald patches were the result of overly-vigorous preening by a prior cage mate. I had him checked at the vet and found out that he had a pretty serious and long-term intestinal yeast infection. He suggested that Max may have been "scratching" at the irritation caused by the infection, and that may have produced the bald patches. Unfortunately, the feather follicles appear to have been permanently damaged, so Max is likely to remain, like Daddy, a little bald.
Maxwell doesn't appear to be plucking feathers. It is possible that he CHEWS them, however, especially newly erupted pin feathers. I've never seen him do it, but he has some damage on a new flight feather that looks like it has been chewed. Chewing can cause the bald spots too, but won't leave any tell-tale feathers laying on the bottom of the cage.
I wouldn't be TOO concerned about your bird's weight, as long as there aren't any sudden, radical changes. Maxwell is visibly smaller than most Senegals I see, and his lower weight is a reflection of his smaller frame. After two years, with Max having access to all the pellets he can eat (and more almonds than he should), he's never gotten above 114g. Based on an (admittedly small) data set I've found showing weights for Senegal parrots, both our birds are within two standard deviations of average and the range of variation of weight is somewhere around 60g. Some Senegals just weigh up to 50% more than others, and speaking as a data analyst (and not an avian vet) I'd say based on the raw numbers, 110-115g is low, but reasonably normal. Your bird may not be undernourished; she may just be petite!