I think the best solution has been said before matches need to be made outside of the player control for tournament settings. Something like duty finder would be best. That way everyone playing in the roulette would be gunning for a win. It would be impossible to know in advance who you would be playing or strategise a way to beat the deck. There should be no forms of bonus of any kind, simply win, draw, loss. The roulette would take care to make sure you play players within a reasonable score of your cards (say rated 1 point for every star). If a player has rating of 200 from 51 cards you could be matched with 180-220 etc. You could not complain then being matched against 3 star decks all the time..
As you can tell the problem with the tournament is incredibly complex, or you could just remove the MGP bonus and then everyone would stop playing once they have the cards and the tournament would die.
