Well I don't know about that program or multiplayer/ network security but a very simple yet maybe effective check on the server could be if the entity moved more than his maximum speed in the last frames (the server should then of course save the old position of every player).


I don't know what you mean, could you explain in detail?
Sending something like
PL_INFO player_info[16];
could be realized as simple as
for(i = 0; i < max_player; i++) send_data_to(NULL,player_info[i],sizeof(PL_INFO));