In my opinion the legs doesn't have to be used in fighter mode since it's covered up by the belly plates. That frees up the space inside for some beefy arms. Transforming could be as easy as popping off the belly plates, swing the arms out, attach the legs and then swap the nose cone for a Battroid accurate one. Then attach the belly plates on the thighs. There should be 2 sets of belly plates since the Battroid ones are drawn to be more of a skewed parallelogram instead of a rectangle.
As for the 19, I think the feet are too big. They should have gave us a set of smaller feet that is also more opened up for the jet exhaust.