A lot of what you are asking does not fall under stuff I can answer. The whole idea of the elements and their use in the BIONICLE universe predates my involvement in BIONICLE, so I have no idea what was intended by the line’s creators. At best, I would just be speculating.
My own view on things is that, as with most things, the meaning of the elements got mythologized by the Matoran. If I build six artificial beings and give one the ability to control fire, one the ability to control water, etc., that’s something that seems pretty basic to me in a science-fantasy universe. Matoran and Toa were given affinity with certain elements because it helped them fulfill their roles inside the robot. I never saw it as going deeper than that.