Plisken nailed it, if the DE Webway carrier is killed, bye-bye portal.
To answer your second question, the Raiders enter play from the portal, which is in effect 'the board edge', and thus can move any distance you like.
The portals are great for running the Raiders out 12", disembarking 2", FoF d6", and then charging 6". In any event, the Raider counts as moving more than 6".
You might be confusing this with the Screaming Jets upgrade, which allows Raiders to Deep Strike, where they count as moving more than 6".