or the person making the FMV in the final version was lazy and just cut a corner by not making Selphie a new shadow that shows up for less than 10 seconds.

Rinoa is in the trial version because she is in the logo of FFVIII. If she was not in the demo somewhere, then many fans of the series would be pressuring Square for more info or another demo, which would not be desirable for making a game at all.