> Attack will be from outside of the ecliptic plane, so likely undetected.

It won't be. There is no stealth in space, and telescopes are really good. Once there will be a lot of man-made objects in space, it will be trivial for a state actor to continuously track the exact position (and complete history) of every single man-made object large enough to have an engine in the solar system, and I would assume that everyone with major space interests would do that.