[1] There is only a 1/18 chance of watcher seeing lawyer on ninja N1 (1/3 setups * 1/6 people), so that will only be relevant once every 18 games or so (in fact, it's lower if you factor in the 1/6 chance of mafia shooting watcher N1)
[2] There are multiple ways for the mafia to counter the watcher: someone CCing watcher, lawyer claiming cop who investigated ninja, claiming tracker and that watcher doesn't exist, or claiming tracker and cop doesn't exist... etc
[3] Since the mafia know the setup N1, if it is the lawyer setup then it will encourage the mafia to CC, since there is the possibility of watcher catching both mafia N1, as you said. Of course, if the cop doesn't out day 1, then the watcher may believe his visit is the cop or tracker visit.
TheeCake