EndofMysteries:
I checked the Hebrew at those places you mentioned and this is what I found:
Ex 18:11, "than all the gods", with the article before elohim (mikkol-ha'elohim).
Ex 18:12, "and sacrifices to God", with preposition before elohim (uzavachim le'lohim).
Ex 18:12b "before the God", with art. (liphne ha'elohim).
Ex 18:16, "statutes of the God", with the article before elohim ('eth-chuqqe ha'elohim).
As noted, both the NWT, Watts, and the Concordant Literal Bible acknowledge the significance of the article in those places where it appears, by adding "true," "the One True," "the One, Elohim," respectively.