May the tower use the runway while an emergency aircraft is inbound?
FAA Order JO 7110.65W - ATC Procedures, Chapter 10 outlines how ATC should handle procedures for emergency situations

Right at he very beginning, we see the most important part:


  10-1-1(d)
  Because of the infinite variety of possible emergency situations, specific procedures cannot be prescribed. However, when you believe an emergency exists or is imminent, select and pursue a course of action which appears to be most appropriate under the circumstances and which most nearly conforms to the instructions in this manual.


In other words, they don't dictate what controllers should do in an emergency; they are to use their best judgment about how to handle it.

So, a controller is free to continue using a runway for both arrivals and departures if they believe doing so won't interfere with the emergency aircraft.

In fact, for non-urgent emergencies like gear extension failures, I've heard them hold the emergency aircraft to get as many others in and out as possible before the runway is closed for a gear-up landing.