This is my way of weaving the entire story together:

Both
Kirkland twins are RJ. They have gotten plastic surgery together, so
they don't look the same as they used to, but they still look identical
to each other. This is why Kirkland asks Lennon (who used to bring RJ
girls YEARS ago) whether he recognized him in the hospital. The
Kirklands want to make sure their new face is unrecognizable even to
their old followers.

The Kirklands know that Jane is closing in.
They know he has the list. What better way to remove themselves from
suspicion than having one of them "interrogate" and kill suspects from
the fake list, under the pretense he is hunting RJ for murdering his
twin? Remember, Jane knows that RJ has the real list. So someone like
Kirkland who is willing to torture and kill people on the fake list
cannot possibly be RJ. This works so well that by the time Kirkland is
arrested (and before he is killed by Smith), Jane has already "narrowed"
the list down to 5 suspects. Since the "RJ murdered my twin" story is
just a cover, this explains why the Kirkland at the hospital didn't
bother to interrogate Lennon about RJ.

RJ is most likely the
leader of Tyger Tyger. The Kirklands decide to have one of them approach
Reede Smith and start poking around Tyger Tyger in the wrong way, such
that Reede's suspicions are aroused. Reede goes back to RJ, the TT
leader (though Reede doesn't know it's the Kirklands), and RJ tells him
to get rid of the "agent" Kirkland. So now Jane thinks that Kirkland is
dead and can't be RJ, and the members of TT are also none the wiser as
to RJ's true identity. Obviously the Kirklands must have decided ahead
of time that one of them would have to sacrifice himself (more likely
the weaker-willed one).

This is sort of the plot of The Prestige -
a pair of twins are living life under a single identity, with each of
them taking turns to live different parts of the life. When one of them
loses a finger in an accident, the other one has to chop his own finger
off to maintain an identical appearance (hence my theory about the
Kirklands getting plastic surgery together to remain identical). In the
end, one of the twins sacrifices himself and dies so the other can go
free (hence my theory about the Kirklands deciding that one of them will
have to sacrifice themselves and be shot by Reede).

Now some of you might think this is incredibly far fetched, but let's face it, Red John is a bona fide criminal genius. My theory is probably one of the few schemes that we would consider worthy of Red John.

Speaking of no loose ends - Kirkland's voice matches RJ's almost perfectly.