well the merc's limit is 20 rockets, but he can sit on a depot. as far as lag, its client side for the debris. All the server does is keep track of the position of the rocket, then the explosion is told to all the clients within potential hearable range (about 500 units) if its not in line of sight (so yes, you can see them through walls).
The tweaks to it are going to be last. Also have to keep in mind that the class limit for merc is one per server, not one per team. After each death he does an autojoin command (ok its a hack but it works) and will join the team with the less points if even, the team with the less players. So he could be switching at each death.
