In "The Case for the Defence," how does suspense persist even at the end?

In this story, the suspense continues even after the story has ended because we do not know if Mrs. Salmon (and perhaps others) might be in danger from the murderer.

At the end of the story, one of the Adams twins is run over by a bus and killed.  Of course, we do not know if it was the killer or the other brother.  The living brother looks over the dead brother’s body, straight at Mrs. Salmon.  The story ends with the line

“But if you were Mrs. Salmon, could you sleep at night?” 

This leaves us in suspense.  We do not know which brother lived and which died and we do not know if, by looking at Mrs. Salmon, he is showing that he intends to kill her.

How does "The Case for the Defense" by Graham Greene build suspense?

I think that Greene builds up suspense in two separate ways.  The first way is through the intrinsic drama of the legal proceedings.  Repeatedly, the narrator suggests that the trial itself is not a "big deal."  It is seen as something almost a matter of fact.  In this, Greene builds up suspense through the moment in which the trial turns into something "extraordinary."  The cross examination of Mrs. Salmon is a moment where suspense is built and suddenly the ground upon which the reader felt was certain is far from it: 

Counsel took a look round the court for a moment. Then he said, `Do you mind, Mrs Salmon, examining again the people in court? No, not the prisoner. Stand up, please, Mr Adams,' and there at the back of the court with thick stout body and muscular legs and a pair of bulging eyes, was the exact image of the man in the dock. `Now think very carefully, Mrs Salmon. Can you still swear that the man you saw drop the hammer in Mrs Parker's garden was the prisoner - and not this man, who is his twin brother?'  Of course she couldn't. She looked from one to the other and didn't say a word.

I think that this helps to enhance the suspense in the court because it completely shifts the focus of the trial.  From something that was framed as more procedural, there is a clear demarcation where uncertainty and doubt has emerged.  Once Mrs. Salmon, the supposed bedrock of all witness testimony changes her story, the rest of the witnesses follow suit.  In this dramatic moment, suspense is heightened from a legal standpoint.

I think that the second way in which suspense is built up happens outside of the court room. The hustle and bustle outside of the courtroom is filled with an element of suspense.  There is the desire to whisk both brothers out the back way of the courthouse, to which one protests by saying he was exonerated.  The crowd of people, the chaos still brought on by the seismic shift of the court proceedings, as well as the fact that some element that might seek to embrace action outside of the courtroom all conspire to bring out the dramatic ending:

Then it happened. I don't know how, though I was only six feet away. The crowd moved and somehow one of the twins got pushed on to the road right in front of a bus.

It is here where the suspense reaches its most intense moment in the story.  Through this, Greene is able to build up suspense both in the courtroom and outside of it.  As a result, there is a great intensity of drama and suspense in the short story as it takes place in two different realms, contributing to an overall suspenseful effect in the reader.

How does Greene maintain suspense until the end in "The Case for the Defense"?

I think that Greene is able to sustain the suspense feel of the short story because of the construction he offers.  Greene is able to develop the feel of suspense both in the courtroom and outside of it.  In operating through two realms, Greene is able to develop and sustain the feel of suspense in both worlds.  In the courtroom, the stunning reversal of Mrs. Salmon as a witness as well as all of the eyewitness testimony against the accused is powerful and filled with suspense.  This is continued outside the courtroom.  The scene after the trial is unclear, one in which there is chaos and little in way of certainty.  The fact that "it" happens without a sense of design, control, or even perceived premeditation is a part of this suspense.  In about five minutes, Mr. Adams goes from the thrill of exoneration to confronting or experiencing death, as we are uncertain which Adams dies at the wheels of the bus.  It is through this where Greene is able to continue the drive for suspense as the events of the courtroom spill into life outside of it.  In the idea of being able to challenge the reader's expectations in both realms, suspense is evident.