How to prevent other elements from an allegorical film from being misinterpreted? Let's say you make a film where the character is in a dark world and runs after a light. The light is the end of the tunnel, where the end is death. The dark world is the tunnel. How do you prevent other elements of the movie from impacting this allegorical symbolism? Let's say the guy in the middle of the film walks into a bright room. How do you prevent that scene and other similar scenes from going against the true allegorical meaning of the film? What about the scene where the guy dreams and is reborn as a kid? This would completely go against the intended allegorical meaning, how do you tell your audience to ignore these scenes or details?
You can't. Nor shouldn't.
If you want to be symbolic, you should stick with it to the end. Taking out the subtleness to tell the readers straightforward what truly happened can kill the discussion and mystery around the story/movie completely.
Though a lot of authors had made similar stuff both "in" the source (by including unnecessary scenes explaining certain stuff that was better left to interpretation) as well as "out" of it, answering the doubts the readers had through interviews. This is actually a trope called the Word of God.
Then you have cases like, for example, the novel Let the Right One In by John Ajvide Lindqvist. It has a friendship/puppy-romance between a young boy, Oskar, and a vampire girl, Eli. A lot of people interpreted Eli's intentions with Oskar as malignant, even one of the movie adaptations (The American movie Let me In) did, despite the fact the author didn't mean it like that.
So he literally went out of his way to create a short story about love - true love - called Let the Old Dreams Die, leaving any doubts behind that Eli really liked Oskar.
In your particular case have the movie as a whole get brighter over the course of the movie would let you carry the theme via a wider angle than just the lone light. You can still have arriving at the light be death and in some sense the rest of the film's brightness can kind of represent how close the character is to death (so an extra bright room earlier on could be a close brush but then returning to dimmer areas lets the overall theme stay.
This also can work with the "rebirth dream" where you keep the brightness consistent with where they are in the movie's time frame/closeness to actual death.
Overall though an allegorical story is bound to attract alternate interpretations. Some you can see coming and try to deal with but you'll not notice others. Overall don't just try to cut off all alternate threads but by repeating and expanding on the key imagery (like by having the whole move grow brighter over time) you can reinforce the ones you are trying to get across.
There is no way to control how readers/views will interpret a work. The most thoroughly allegorical work, say The Roman de la Rose or Animal Farm will be read on a purely surface level by some, and the least symbolic work imaginable, say Fun with Dick and Jane or perhaps Galactic Patrol, will be treated as an elaborate allegory by others.
If one writes a thoroughly allegorical work, every piece will fit into a single overall scheme or pattern. That will mean that there is a single "correct answer", by authorial intent. Many readers will correctly interpret this if it is done well, and the "correct answer" will be widely known and generally accepted (if the work is widely read/viewed at all). Animal Farm is surely a case in point. That is one way to avoid, or at least minimize, misinterpretations: have a single intended interpretation into which everything fits neatly. No other interpretation will be as neat and all-encompassing, and so most should "get" your intended solution.
Works with more complex and nuanced symbolism, in which some aspects are clearly symbolic, others are probably not, are fair game for interpreters. Once it is accepted that some elements are symbolic, some readers (or viewers) will interpret other elements in that way. Moreover, it is common for authors to include symbolism that they were not themselves aware of when they created a work. An interesting case in point is to read James Blish's analysis of his own works "Common Time" (short story) and A Case of Conscience (novel). He finds extensive and consistent symbolism that, by his own account, was not consciously intended. Given Blish's own skills as a literary analyst, and the intentional and explicit references to Finnegan's Wake in A Case of Conscience this only shows that even a quite self-aware author may include unintended symbolism.
The point is to make the story first work well on its surface level, and second, make any intended symbolic content illuminate and enhance the story. If others find a symbolic interpretation you did not intend, so be it. Perhaps it will show you something about yourself you did not know. Or perhaps it will merely make you marvel at the way people can find things you never intended in a work, things that may seem truly off the wall. (I recall a detailed analysis of the song "American Pie" that showed it was all about the Kennedy Assassination. The Joker was Lyndon Johnson, as I recall.) As long as you are satisfied with the work, and others have enjoyed or appreciated it, why worry if they don't all take it as you did?