This would be an act of war.
There is no way to know if a "near-miss" is actually an intended hit until it is too late.
Trump would be justified in shooting down every missile that is launched from North Korea from this point further, then taking out the launch sites.
It would be beneficial to have UN backing beforehand, but it is not necessary.
I think people are wildly overestimating our ABM capability. Even in highly controlled recent tests, there hasn't been a great success rate.
And if we attempt and miss, that's not good.
Kim's choice of "near-miss" with conventional warheads at Guam is a great choice. Let's look at our options...
1) Respond by trying to shoot them down.
We'll get partial success, at best. That just reveals our capability level. When the remaining missiles splash down off Guam, he can point that out, and point out that he wasn't firing for effect. Even if we hit them all, jump ahead to our response, below.
2) Don't respond. Looks bad. When they splash down not hitting anything, we have to figure out our response.
For our response, strategic nukes are out. We'd have the condemnation of the world for using strategic nukes on someone who fired conventional warning shots. Plus, we'd have collateral effects on ROK, PRC, and Japan.
Tactical nukes, similar problems, plus would lack total effectiveness.
Conventional response. Oh, boy. At best, we do a bit of damage. But again, Kim can point out that we responded with live fire to his warning "tests". What does it really do for us? Watch for video of all the collateral damage, as weak-minded world opinion turns against us.
I hope Trump doesn't back himself into a corner.