It is my understanding that since all that is, was created by God, there couldn't be anything that He is not. Now when it comes to things that aren't things, like terrible actions of humankind, even when I act completely against what I might consider to be God's will, He gave me the choice to do so and I wouldn't be without Him... in this sense, nothing exists that is not of God ---not to imply that when I choose to do terrible things, that's what my Creator wants... in a sense, all that is good comes from God and all that is bad comes from us... but he made us and gave us a choice, so in that sense yes even when we act against our spiritual compass, we are exercising the free will that we wouldn't have if He didn't give it to us.
He is All-Present, so that includes everywhere and much more that may not be a place.
He is not a person, but His will has been revealed through people time and time again throughout the ages.
He is One because by definition of His attributes (Most Powerful etc.) there can not be more than one.
This of course is only my personal understanding and as such well prone to errors and misunderstanding.