Hi Scorp
Lane's Arabic-English Lexicon (which is based on classical Arabic dictionaries), says under the word Allah, while citing many linguistical authorities:
"Allah ... is a proper name applied to the Being Who exists necessarily, by Himself, comprising all the attributes of perfection, a proper name denoting the true god ... the "al" being inseparable from it, not derived..."
Allah is thus a proper name, not derived from anything, and the Al is inseparable from it. The word al-ilah (the god) is a different word.
Thx
Alexei