Great idea, was thinking about that, BUT:
The spy itself is really skinny and doesn't have a very huge muscle mass, and for a mercenary that is bulky and heavy, would be pretty ridicilously easy to get out of the neckbreak position, especially if he decides to put more force back(The Mercenary) and crush the spy with his heavy mass.
Other than that, I have no objections, but the spy definately should not be able to:
A) Move.
B) Taking out tazer should at least take 3 seconds after the grab to come out.
C) It takes couple of seconds to necksnap after putting away the tazer.
D) The mercenary should choke faster for the balance issue, and cannot be/CAN ONLY? knocked out.