Well, the Herald isn't technically dead.
Not yet, and 'yet' is the important word here.
As Shay says, temporal dilation means that the process of the Herald being crushed into the singularity will take some indeterminate amount of time that's probably greater than the universe's current age squared. This is not any exaggeration either; being so close to that much mass makes space-time absurdly warped to the point that time is near to stopped. In a very real sense, to the outside cosmos, all black holes are forever in the process of collapsing into their singularities, with the inrushing mass frozen in the slowest of slow motions as everything is dragged to the core.
A nanosecond inside is equal to a literal eternity outside.
However ... the Herald's fate is now sealed, and it cannot influence anything outside of where it is. So, though it's not dead *yet*, it has effectively been removed from the equation.
Given that the alien swarm was already following the Herald's specific steps to attack Earth even as it went absent to chase Shay and friends, there's a good chance nothing major will immediately change after the snake's head is removed. The body -- the attacking armada -- has instructions and knows where, when, and how to act.
As for the 'spy mission', I'm assuming you're referring to Iskandar meeting Lucas. That just didn't feature this chapter, but it will soon.
Not going to say anything else about the Master, because at this point there is nothing for me to say.
If your questions are ones that I can provide any commentary or insight on, feel free to ask away!