"The DART spacecraft changed the orbital period of Dimorphos by a whopping 32 minutes"

[Bernardo de La Paz]

4. No. It is in orbit around a second asteroid. A real problem asteroid would get more mass

The technique has been proven, now.

If there was a real problem asteroid in our future, it would be given a much more massive hit. And perhaps multiple hits.

The mass of the impacter was 570 kg (1260 pounds for provincials). It would not be difficult to assemble in a few months a fleet of say a dozen boosters and their impacters massing say 10,000 kg each (22,000 pounds). 120,000 kg / 570 kg = 210 times the mass.

Plus, you can by choosing timing and orbital insertions and gravity boosts you can gain velocity and hence momentum at impact.

Plus,

The other big thing is that the farther ahead you can determine the need, the bigger end effect a nudge has. So in principle if you detect it far enough ahead, you could impact it with a feather at a walk and it be enough. That is effectively impossible because computing the orbits is a "multi-body problem" where computing even three bodies can be a challenge.

So, is it enough mass? Not this one perhaps, given anticipated forecasting and sizes involved, but something like it is enough mass. Search and forecasting are improving too.