There are three issues with your premises: Piccolo's ki is implied to have decreased by Cell, Cell's didn't. You are using an instance in which Piccolo is still at the upper hand and literally states that he's still more powerful than him, in contrast, you admit that Cell still had the upperhand against Vegeta. In the case of Gohan's kamehameha against Cell, Cell questions why does Gohan have so much power, Cell against Vegeta was smiling and saying that he is joking. Your comment is... ugh... it doesn't make any sense at all.