We find in Matthew 4:9, that the devil said to Jesus, “All these (kingdoms of the world) I will bestow on you if you prostrate yourself in homage before me.”
It seems the devil does have dominion over all the earth, to the extent that God permits it. But God is sovereign over all, and brings good out of evil, so we can always trust Him, for even the hairs of our head are numbered.
As far as the weeds being mingled with the wheat, the reason Jesus said to let them grow together, is that there is so little difference between the two that one may mistake a good plant for a weed. There are photos on the internet of both darnel and wheat, so you can see the difference. A person would have difficulty distinguishing which is the good plant, and might uproot it by mistake. It is said that one cannot tell the difference until it is fully grown.