The Bookmark field could never be a calculated field, as it is a standard field and has a lot of coding associated with it. Clearing it after a certain time would be difficult as well I think. MC doesn't really allow that sort of field update by separate expressions.
Did you know that pressing Stop twice clears the Bookmark? So if you played a move and it started from a Bookmark, just press Stop twice, then Play again, and MC will start from the beginning.
The Showtime functionality could perhaps be modified to always start a movie from the beginning. That sort of makes sense as a user who wanted to go through the Showtime feature before the movie is likely into the whole "Show me everything" mindset.
The next easiest solution I think is the idea in your second post. There is an
MCC 10001, which is just called "MCC_PLAY", but I think it starts playing from the beginning of a file.
There is a table at the bottom of the page I linked to that calls the operation of 10001 "Play track from start". Test it and see. It may or may not play from the start for a video file.