If they had detected it, would there have been enough advanced warning to clear people out of the target area?

Completely depends on how far ahead we got notice. Most objects we should be able to detect well ahead of time from other close passes, and then we just know in blank years it will be back along this place where the Earth crosses. But with long-period objects, we really can't do that. Long period objects come from the kuiper belt or even further out from the oort cloud. Think like Haley's or Hale-Bopp Comets. Some of those only come into the inner solar system once a lifetime, some we only see once. Those make predicting objects very difficult, because if we don't see it coming the first time, that might be the only chance we have.

I'm sure they can invent/create devices that would scan the sky automatically and look for movements, but they would need a lot of them, and they would need to be dedicated to the task of finding these objects. Place them at various lagrange points, as well as a few leading and following the Earth around the sun. Right now many telescopes and devices used for looking for NEOs aren't dedicated, they are "shared science projects". So one week they might be looking for Earth killing asteroids, the next week another team might be using them to study pulsars.