Yes God is omnipresent. But you seem to have forgotten that until the day of Pentecost in 30 AD, the Holy Spirit had not been given as a gift to those who believe. He had not be sent down to earth by the Father. And just as He came down to earth for the Church Age He will return to Heaven when the Church is raptured and to allow Satan full control. As to an angel being the Restrainer mentioned in 2 Thessalonians, that is just laughable. Even Michael the archangel said to Satan "the Lord rebuke thee".
You say it’s “laughable” that an angel would be able to restrain the Man of Sin. Why? He is just a man. Even one who, at some point, may have certain powers given to him by Satan.
In scripture we have plenty of ideas of conflict between spiritual beings. The primary one would be in Daniel, where the angel who is sent to give Daniel a message is held up by a “Prince”…and needs Michael to come to his assistance.
This tells us that an ‘angelic’ battle rages behind the scenes, so to speak. And while, yes, we know Michael would not use his own authority to rebuke Satan, that does not give us a full picture of what this other place, or its structures look like.
One ought to keep in mind that God’s purposes will be done, and if he gives authority to someone like Michael to do it, no one, not even Satan would stand in his way. All power, Michaels…even Satan’s, ultimately, comes from God.
If you’re interested in digging into that topic a little more, do a search on “divine council”. Or read Enoch.