There is an easy way to do this: There are two different actions in Lights & Switches that apply. “Turn on when there is motion”, and “Turn off when there is no motion”. What you do is turn your lights on with “Turn on when there is motion”, but do not select the option for “Turn off when motion stops”. Instead what you do, is use “Turn off when there is no motion” to turn them off after 15 minutes. Place the mode restriction on the latter, to not turn them off if mode is “movie time”.
So, they come on as you describe, and will stay on for 15 minutes. But, in the interim mode changes to “movie time”. Now, “Turn off when there is no motion” will not run, because it’s the wrong mode, and they won’t turn off.