I think this is the behavior of your set-top player. If you press Stop once, the player still remember the last position that you stop and resumes playback, with the last selected audio/subtitle track, when you press Play again. But if you press Stop twice, it could start the movie from the beginning. At least this is how it behaves on my settop players even if my dvd has a commentary track.
Is there audio track 1 and 2? Did you modify the DVD in any way?
You cannot program the Play button to go to the main menu. On your remote, there should be a Menu (or Root) button. This will go to the main menu.
To understand DVD structure and commands, the manual would be a good start. Check also the Appendix section. Also the new debug feature in DRMP will help you understand how DVD is navigated, using Step button to trace step-by-step. The Command Edit dialog also provides a brief description of what each command type does. One good site you want to read is http://www.dvd-replica.com/DVD/index.php. If the first time you don't understand as there are lots of terminologies to absorb, read again and again. You will one day understand how it works.