Great, first post and already legal stuff:
1) The process of reverse enginieering software is not illegal at all. You create a piece of software on your own in that process. However, the resulting software may get you into some IP or patent trouble.
2) If the resulting reverse engineered server is not providing the IP but the IP is stored solely in the game clients of the connecting users, then the server does neither break copyright nor IP related laws. The IP was received via NCsoft's download servers and is not "completed" via the reverse engineered server, but brought back to be working after being disfunct by the manufacturer. In software, compare to the running and unprosecuted project SWGEmu, an emulation server for the disfunct MMO Star Wars Galaxies. In general this process can be compared to rebuilding a spare part for a classic car or a WW2 fighter (like the P51) that the original manufacturer cannot or does not want to provide any longer.
3) If the patents concerning the way game mechanics are too generic, lawsuits may get turned down - keep an eye on the lawsuit Worlds Inc. vs Blizzard for that one.
4) To prove that the reverse engineered server works in a to similar way compared to the original server software, NCsoft would need to reveal their source code.
5) Success of any C&D or comparable action highly depends on the location of the server, and you don't have to move the server to Uganda or Uruguay to gain a massive advantage: Germany alone would do, and in the same move takes care of the success concerning IP and copyright problems as well. Ask any frustrated lawyer of EA about this.
So basically, if the process running behind what we call the loading screen isn't much more than registering our chat-handles from local.ap to local.kr (e.g.) and copying information from the clients' HDD/SDD to the clients' RAM, there won't be much NCsoft could do against the RE'd server. It will just turn a lil more complicated if the server is adding IP-containing information to the clients' systems - if you don#t move to a zone with more user-orientated laws