After a trial run through this scenario, I'm finding that we need a game rule to cover what effect Jaffar's cards have when Sinbad has not yet played any cards that could potentially be cancelled.

Possibility #1: The cards do nothing. Jaffar therefore can't really hinder Sinbad until Sinbad begins to play cards.

Possibility #2: Jaffar can place these cards so as to "block" the available challenge paths. To unblock the path and begin working along it, Sinbad must first play the correct card type to counter Jaffar's blocking card (Muscle - Magic; Friends - Mind Control; Dumb Luck - Supplications).

I favor the second rule myself, especially for games in which a second player takes the role of Jaffar. It gives that player something to do, as well as introducing an element of strategy by allowing Jaffar to choose which path/s he wants to use his cards to block. If no one is playing Jaffar, Jaffar's cards will always block the shortest paths first.

I also like the second rule because without it, the strategy for beginning each challenge is a no-brainer (perhaps appropriately, as this is Sinbad we're talking about here). You simply sit back and take advantage of the opportunity to draw 2 cards on each of your first 2 turns, while Jaffar is still Losing. Then beginning on the third turn, you deploy your stockpiled resources. If there's a race to begin a path before Jaffar can block it, this strategy isn't so straightforward.

I also also like the second rule because it prevents Sinbad from breezing through the shorter challenges, such as those found in the first Basra scenario. With longer challenges, the problem takes care of itself, as Jaffar has plenty of opportunities to hinder Sinbad's efforts to finish the challenge.