If Oswald didn’t shoot Kennedy someone else will have

Cambridge Grammar of the English Language, page 200, reads

If Oswald didn’t shoot Kennedy someone else will have would be appropriate in a context where it was not known that Kennedy was shot.

I cannot understand what this paragraphs tries to explain.

***EDIT: I think there might be a typo, so it'd be hasn't shot instead of didn't shoot. The context is a plot with another assassins in case Oswald didn't succeedm, and the sentence itself is uttered by someone who knows about the plot and it's time of execution -the sentence being uttered after such a time.

Solution 1:

If Oswald didn't shoot Kennedy someone else will have.

Your edit provides the context for this very bizarre statement.

They're employing the future perfect tense into the passive voice and doing it to convey a sense of possible different outcomes.

That's a mouthful. So, instead let me illustrate it for you:

Assassination plotter (AP) A says: What do you think happened? Do you think Kennedy is dead?

AP B says: We hired a bunch of shooters for this job. If Oswald didn't shoot Kennedy someone else will have.

So, we know that there was a planned assassination attempt. We don't know for sure that it went off correctly. But, we do know that the assassins had a backup plan.

If (the condition arises where) Oswald didn't shoot Kennedy (by the time of the planned assassination -- indicated by the past tense didn't shoot) someone else will have (done it instead of Oswald indicated by the future perfect tense will have).

This type of difficulty parsing the statement is why many style manuals warn against using the passive voice.

You could make this whole thing clearer by saying: