The conventions of movies help disguise the fact that Nash's delusions and hallucinations aren't real. Things that really don't make sense aren't questioned because we, the audience (or at least I, the person writing this) are used to them happening in movies, so we get pulled along with the story.
Marcie never ages. In a two or so hour movie, with dates mentioned but not focused on, this isn't strange, all other characters look pretty similar over the time period in question. But Marcie as a younger kid would have changed a bit more in the several years over which the first part of the movie happens.
We never see how Nash imagines Charles' living arrangements in a single person room, because it is a movie and movies only show important snippet's of people's life.
Nash's interactions with Parcher follow a number of spy movie cliches, and also has more movie features such as the soundtrack more in the forefront. This also matches how actual hallucinations and conspiracy theories make the world scarier, or more exciting, and/or make the person a part of a bigger story than they otherwise would be. When Nash goes back to teaching and interacting with people at Princeton, the movie gets noticeably more everyday life feeling, with more ordinary interactions.
Being asked to find hidden messages in ordinary books, newspapers, magazines, and other media is a common conspiracy/pseudoscience/paranormal/etc. thing. But because we've seen several involved scenes with Parcher, and are hit with lots of odd details to take in as is normal in these movies, asking for codes in regular media in 1-2 quick sentences doesn't register as weird.
