Hens do naturally lose quite a bit of weight when molting. It depends on the individual hen but some of mine lose only a little bit and others lose a lot, despite eating. Give her complete access to her feed, and try to encourage her to eat more protein.
It's tough when you really don't know what's going on but if she seems to be acting fine (preening, dustbathing, foraging for greens, standing up for herself in the pecking order, coming for treats, etc.) then the best you can do is just make sure she eats, and eats well. Again,
complete access to feed is very important for them at the molting stage!
If you're having trouble holding her to examine the breastbone, molting will actually make them even more unfriendly! All of the pinfeathers are very sensitive and it hurts to be caught and held! I always check the more unfriendly hens at night when they've gone up on their perch. The darkness kinda puts them in a stupor because they really are blind at night, so I can pretty much feel everything. I always check their crop to see how much they ate, their breastbone to see if they've lost any weight, and their abdomen as well.
Don't feel bad! It's all a learning process and I can see you're trying your best to understand what's going on! If you think you can safely rule out parasites and/or infection, then this is probably a molt. The best bet is extra food and TLC

.
P.S. Also, hate to be nosey...but what feed do you give them and is it cold where you're at right now? If it's cold, then a molting hen will need A LOT more food to make up for all the calories she's burning trying to molt
and keep warm at the same time. I know! It's all so complicated but once you get it down pat, you're golden!