it is a trick question.
If Allah is God and if there is a God, by definition he/she/it cannot 'exist'.
Existence only makes sense when there is non-existence. God by definition is above all that crap. Because when you say a glass is full, you are also saying it can be, could have been and could become empty or unfull.
To go one step further,
Anything that exists cannot be God, because God cannot be bound by rules of existence and non-existence.
Which is why the Hindu understanding of God makes a lot of sense which is you are God. If you are not, God is not. The acknowledgement of things most personal
Peace
The primary elements to your reasoning are sound then you make major errors in conclusion.
In fact if you look in Islamic scriptures it clearly states that God is One, and God is First and so on, but it's really only people who say "before God there was nothing" which is a fallacious statement because it implies that 'before God' has a tangible meaning, which if God is First it does not carry that meaning.
But then to conclude that anything that exists cannot be God is also wrong but existence has a common understanding and it often used in a convenient manner. When we say "Does God Exist?" We really only use the phrase out of convenience not out of something that can be reasoned philosophically in and of itself.
Rather you should conclude that therefore we should not say that God exists in the sense that we exist and leave it at that.
You said:
Which is why the Hindu understanding of God makes a lot of sense which is you are God. If you are not, God is not.
This statement is not superior to the Islamic understanding of God. Because when there was nothing according to your definition 'if you are not, God is not' then it is possible for God to be not. This in itself is as bad as saying that God exists.
As I said ... God is First ... to say anything else is wrong and cannot be reasoned. In other words to assume that God cannot be i.e. for there to be 0 Gods, or for there to be > 1 God is subject to error and theological problems.