In Muslim faith, is there no depiction of Allah just as there is no allowed picture of Mohammed?

Yes. We do not encourage IDOL worship. Allah is the most high. The God of Everything whereas Jesus, Muhammed, Adam are all his messengers. All came with the same message. Unfortunately people misunderstood them and became the sects.

Even if today if Muslim, Jews and Christianity agree the last prophet was Muhammed(PBUH) just like we agree that Jesus and Moses is our prophet as well. Then there wouldn't be any place for this atheist.

Part of the 'Test of Life' is to accept and believe in a Creator without any 'depiction'.

So, YES for the first part.

And since there is no way for 'real' depiction, any imaginary depiction of any of the prophets, especially that of Prophet Muhammad, being the 'Seal of prophets' is prohibited in Islam, for fear that would lead to idolatry, a grave unforgivable sin in Islam. An example is the Christians making Jesus, a man into a God and are 'hell bound' because of this.

Yes thats true. God cannot be depicted. No human attributes can be applied to Him. Similarly, no depiction can be made of any of the Prophets of God in the Islamic faith (Muhammad, Jesus, Moses, Abraham etc, peace be upon them all).

yes

that is correct. that is also correct of judaism and christianity