Did the Second Coming already happen in the first century, so that we are now in Satan's 'little season'?

No. For one thing, the little season is not a big season; the little season of Apocalypse 20:3 comes after the thousand years and is short by comparison, run by the man of sin, the antichrist of 2 Thessalonians 2, and everything in Scripture says it will be brief. We are living in the thousand years now, the millennium from the cross to the Second Coming, so the little season comes just before Christ returns, not now. The Second Coming has not happened; it is still future.