I imagine it would be possible to construct the system to substantially reduce the risk, even reduce it to "cosmic rays cause bit-flips which launch nukes" levels of risk. It's not like military nuclear command-and-control, you don't need one person who can push the button at any time. If we spot an asteroid, we'll have some time to deal with it. The system would presumably need to be constructed, which would take a few days even if everything was in storage on standby. Then, to activate it, you could have something like the US nuclear weapons' PAL, but instead of the codes being in the hands of one person, you spread them out. Just before launch it's activated with something analogous to the ICANN key ceremony: a bunch of well-regarded astronomers, rocket scientists, and aerospace engineers use their section of the Earth Asteroid Defence Key, and they press the metaphorical button together.
