>There is no exit from the JR lines that does not have clear signage to every other line
Several times I have come across a station (like Shinjuku) where I can see the same sign for the same train line placed in 4 or 5 different areas (all visible at once) all pointing in different directions.
that's because both directions will get you to the train you're trying to get to. Example: From JR either west exit (then turn right) or the east exit (then turn left) will lead to the Marunouchi line. Pick one and follow the signs. That's no different than a subway with multiple exits on to the same street at both ends of the station
Similarly there's at least 3 perpendicular corridors connecting the JR tracks themselves so the exit to each corridor (of which there are 2 each so 6 exits) will all be labeled as leading to the other tracks because all of them actually do lead to the other tracks.
Several times I have come across a station (like Shinjuku) where I can see the same sign for the same train line placed in 4 or 5 different areas (all visible at once) all pointing in different directions.