The seed of Satan has not arrived yet. They will be many and they will be the result of those who make themselves a part of Satan's future throne (Wild Beast) AFTER Christs future "definite" 1000 year kingdom ends. (Rev 2:13, Rev 20:7-8)
Happily, many of these "seeds" - who wil have been misled - will come around and respond to the Good News of the Kingdom declaration made by the angel. (Rev 14:6)
Yes, God will entrust an angel (one angel) with declaring the Good News of the Kingdom to ALL of mankind (who will have been resurrected as Jesus promised), no human will be entrusted with declaring that message.
No one today can lift out of Scripture exactly what that Good News message is. It is not there because God has not given it yet and when he does, an angel will receive it.
So any claims to "preaching" a Good News of the Kingdom is a false message. If it is a false message, it is being declared by witting and unwitting false prophets.
Placing WT publications in the hands of persons and starting Bible studies with them in the hopes they will join a religious organization is not "preaching the Good News of the Kingdom."
Rev 14:6 is clear as to WHO will be tasked with declaring that all important message to ALL of the nations. No human entity can preach this message to ALL of the world.
And "the nations" referred to is all persons born since Adam, to include Adam. That means that the Good News message will have to be declared to all of mankind who will have been resurrected. Their numbers will be like the grains of sand on the seashore. They cannot be counted.
Today, the worlds population can be counted. They cannot be counted if all born since Adam are ressurrected into Christs paradise.
The seed of Satan are persons in those future days after Chists kingdom ends who purposefully chosoe not to respond to the Good News declaration of the Kingdom by the angel and who knowingly align themselves with Satan in the maintenance of his Wild Beast who they will believe is invincible.