The following is from my own understanding and knowledge which could be faulty.
God does not have a "need" for anything, He can like one thing and dislike another, but that does not indicate a "need".
Muslims believe that this life is a test, so that people who deserve
Heavenand
Hell can be differentiated from each other. Muslims believe that before the start of this test, they were asked by God whether they want to go through with it or not. They agreed to be tested, therefore according to this covenant, they have to follow God's commands.
If someone does not worship God, God can be merciful and choose to forgive the person, or God (because of the agreed upon test) can choose not to forgive and punish the person. This is not because God has a need for worship, it is because of breaking the promise.
On the matter of God being everywhere, this is not something that Muslim scholars have had a consensus over. We are not given a clear description of God's attributes, so we can't really say for sure. The following are two articles which go in some detail about this question:
http://www.understanding-islam.com/q-and-a/islamic-beliefs/is-god-everywhere-6570http://www.islamreligion.com/articles/2562/where-is-god/On interacting with the apostate devil, as far as I know about the Islamic doctrine, we have absolutely no idea whether such an interaction takes place or not. It is possible, but we have been given no information for it.
Regards,