If the breast bone is prominent, that indicates weight loss or muscle loss (some birds are just naturally leaner than others, so take that into account) which can also happen as a symptom of what is going on in the abdomen. Body condition slowly deteriorates. I suspect the lump you are feeling is the crop, it's usually low on the right side. there are probably contents in there that have not passed. The gizzard is not accessible from outside, you won't be able to feel it. You can try giving the olive oil or coconut oil and massaging that lump with your fingers to see if it will break up and pass. But not emptying completely is not uncommon with reproductive problems, it just slows everything down. Heavy laying breeds are more prone to reproductive problems, but it can happen to any hen, and anytime after the age of two it's not uncommon.