A missile would not change the re-entry time or location: just break the target into many pieces. In the one case where the US used a missile the target broke into many small pieces which mostly burned up on re-entry but I don’t think that would happen with the ISS. Uncontrolled re-entry of a single large object would, I think, be preferable to re-entry of dozens of them.
No agreement would have any effect on the headlines saying “US allows its spacestation to crash on city, killing 800 people”.
Let’s not set the precedent of having the UN make decisions about space.