…your setup looks like you are using an overhead projector. It probably isn’t though…
As for flatness: All film will bend after a while because the light will warm the film and film borders differently. I notice that film I kept in filing sheets are fairly flat. Film that I kept in rolls (for 30+ years) tends to be harder to tame. I also remember the slide shows that my colleagues did after their travels: Slides were projected and after a few seconds would snap out of focus because they would bend because of the heat. Some of the slides simply burned in projectors that used very powerful bulbs, something that should not happen on your light table/copy stand.