First, u need to discard 6 cards, if u are really unlucky u will discard cards with discard. And u will receive portal which summon 2 imps. Infinity value, yea, but 3/2 imp aren't threat and portal blocks one space on board.
Not possible- you start with this card in hand so it'd always be your turn 1 play. Once you complete the quest, I think it's added to your hand. Then it'd be possible to discard it HOWEVER you'd be able to play it so it'd be dumb to play something that would discard it before playing it ...
It would be hilarious if in the process of attempting this quest you actually discard the Nether Portal.
I am not sure on the mechanics, but after you discard 6 cards, then you get the portal card right? And there are several warlock cards that discard 2 cards. So would it not be possible? Also you could be absentminded and discard it the turn after you get it.