While giving the message to the Arabians, why were the ELOHIM termed as Allah? Couldn't the message to the Arabs be continued as from ELOHIM to avoid all confusion as well as to expand upon the already existing old teaching given to the Israelites?

 Names are often changed by people for different reasons. For sure the Israelites knew the name Eloha, but since the Elohim are several, they kept the plural form. And the Arabs apparently chose to focus on one single Eloha, and so they used the name Allah (a mispronunciation of Eloha).