I like your theories! Except it´s not a keel, it´s a lobe/s. The exhibition-type toulouse do have a keel, but it´s that big breast-bone that sticks out in front, just like the keel of a boat. I can´t quite make out if yours has it or not. And as the lobes get bigger when there´s plenty of food, I also wondered if it´s to keep her going in those weeks when she doesn´t eat much...but then, in some breeds the male has them too...! Pretty geese, by the way...are they chinese crosses?Here you see Molly, a Toulouse goose, in the middle with her two kids. She's got a pretty big keel as we call it. After hatching her kids, it was small and it got bigger as she was off the nest.
I have my theories as to why female geese get keels. One is that they serve as more surface area to cover the eggs in the nest. Also, as Mom gets into water, those feathers get wet and with the added warmth of the extra surface area of the keel would keep the eggs warm and moist.