I can't figure out how to say that if one video is on, the others on that same layer should be off.
In your patch, you are testing the incoming value by limiting the range that passes through with a Pass value Actor. This is a good approach if you are getting different ranges from each sensor.
Next, you have a Trigger Value actor which sends a number. You could use this value to test which stream is active (rather than setting the Movie Number).
You might do this with Comparator actors, or send the value to a Selector actor as suggested above... there are many options.
What you might want to control is the Visible input of the Movie Player (if you would like the video to stop and restart each time), or the Intensity of the Projectors (if you would like them to play continually). Using a Selector as suggested is another option where the movies could continue to play and you would simply switch which movie is routed to a single projector (remove the others).
*** Isadora will layer/stack the videos added to a scene in order (left to right, top to bottom) when they have the same layer number. The layer numbers will allow you to break out of this layering if needed. The layering also uses the blend mode selected in the projector, so you can create some complex composites using nothing more than this technique.