So is this a signal?

What can we learn from that? Never shake a man's hand? 

Mary Me wrote:

What can we learn from that? Never shake a man's hand? 

 
We learn that Tobe desperately needs S3!! 

And me!

My theories:

1.  The assassins were mercenaries hired by Mycroft to protect Sherlock from Moriarty.  Mycroft clearly leaves it ambiguous as to how and why they're there.  "If not Moriarty, then who?"  Along with Mycroft's guilt, and his resources, I think he's trying to protect Sherlock.

2.  Moriarty knows of the assassins (but not of their purpose), he believes that they're there to attack/kill Sherlock for the "code" that Sherlock "believes" exists, even though Moriarty knows there's no code, and Sherlock knows there's no code (but pretends that he thinks there's one, anyway, to Moriarty).

3.  Moriarty's snipers have been instructed to kill the assassins if they make contact with Sherlock.  Moriarty is ensuring that Sherlock is stayin' alive, otherwise he can't perform his own fall.  If the assassins really wanted to kill each other, they could have already done it.  My guess is that Moriarty instructed his snipers to kill any suspicious person who tries to make contact with Sherlock, as it jeopardizes Sherlock's life.

I hypothesize you've hit the nail on the head when you say the rooftop Moriarty/Sherlock handshake is the pivot...but I suggest you've got the wrong "handshake" precedent.

How many times in this series have we collectively gasped at a handshake?  There have been several  (including the assassin handshakes you point out...) but the BIG ONE was the final one in Scandal between Sherlock and Irene.

Sherlock moves in close, takes her hand, and says softly, "I took your pulse."  (Right? How could we forget? *swoon*)

SO... on the rooftop, first time Sherlock is on the ledge and asks for a moment of privacy, Moriarty must be curious. "What is Sherlock up to?" he thinks,  but, ever the gamesman, he says "Of course." Moriarty's smirk says he doesn't care how clever Sherlock is, gravity is gravity. But Sherlock hops off the ledge and tells Moriarty that he can, in fact, save his friends. 

And the writers set us up for the handshake:  "You want me to shake hands with you in Hell?  I shall not disappoint you."

THIS is when Moriarty realizes that Sherlock can save his friends and live, becuase Sherlock is going to fake his own death.  Moriarty realizes that Sherlock used this moment of privacy to do something to allow him to appear dead.  To confirm his theory, Moriarty very, very pointedly offers his hand to Sherlock....

AND TAKES HIS PULSE.    Bam.  There's no pulse.  

Moriarty realizes that he's been beaten.  That's why Moriarty kills himself and Sherlock proceeds with his plan to fake his own suicide.