You can't do anything about getting the ruling overturned. The head judge's ruling at any tournament is final. If the judge ruled that Pot of Greed said to discard 2 cards as a cost to play it, that would be the ruling (I would drop from the tournament at that point and speak with the Tournament Organizer about getting a new judge, but that's beside the point).
Last Turn's effect is not entirely negated if Jowgen is on the field. Last Turn attempts to Special Summon a monster, but because of the effect of Jowgen, it fails that particular effect. The rest of the effect is not dependant upon the summon directly. While, obviously, a battle cannot occur if no monster is Special Summoned, Last Turn's End of Turn effect does not depend on that.
Going over Last Turn, step by step.
When Last Turn resolves, all cards in hand and on the field except for the one chosen to remain go to the Graveyard. That is the only immediate effect of Last Turn. The rest of Last Turn's effect follows in sequence, but if any part fails, the rest still attempts to proceed.
So, the next step is a monster is Special Summoned from the opponent's Deck... but Jowgen prevents this from happening, so we move on to the next step.
The monsters battle, but there is no opposing monster so moving on.
At the end of the turn, if only one player controls monsters, that player wins. Any other condition results in a draw.
And that's it. Jowgen's condition certainly makes it easier to win with Last Turn, but it definitely doesn't negate Last Turn.