"With departure time at 6 PM, what time do we have to be back to board the ship?" You should be back at the ship by at least 5pm. "On Board" times should be posted when you leave the ship.
Never having been in a Suite, I can't answer your questions about receiving priority disembarkation for your own excursions. However, I would think that if the ship was docked you can get off any time you want. If you are tendering then ship excursions do have priority - check this out with the Excursion Office.