Thank you, Andrew—though it feels like I’m talking to myself.
Paul is speaking about the threat of division and disruption from people who serve “their own appetites, and by smooth talk and flattery they deceive the hearts of the naive” (Rom. 16:18). If verse 20 is not an interpolation, the point presumably is that Satan is the driving force behind such opposition (cf. 2 Thess. 2:9), but he will soon be crushed under their feet.
I think it is right to locate this within the eschatological time-frame of the early church in the Roman world. Rome is the satanically inspired “beast” that makes war against the saints (cf. Dan. 7:23-25; Rev. 13:4). In John’s visionary narrative it seems to me that with the defeat of pagan Rome by the testimony of the persecuted churches comes the confinement of Satan to the abyss, not to deceive the nations in this way again until the end of the thousand years (Rev. 20:1-3). By my (apocalyptic) calculation, Satan is still imprisoned and cannot pose such an extreme and total existential threat to the churches again.