I am not 100% sure, is it possible to put an "interactive video" inside a fluid box ?.
We are going to do a multi-device project and the usefulness of "interactive video" is interesting.
AFAIK the answer is NO. Interactive video is always slide video, never event video. You can have event video in a fluid box, but not slide video.
Insert the interactive video on a slide without fluid boxes, and use the Position Properties panel to define the location and the size. Since you need to keep the width/height ratio, define only the width (maybe 100%) and set the height to Auto. It means that the experience of that slide will be optimal in landscape, not in portrait mode. Maybe you'll have to tell the user that information.