The shortage of pumpkins *COULD* be trying to enforce the concept that the goal is NOT to be able to get *EVERYTHING*. I understand some games are designed such that everyone can get everything during an event. But it is also reasonable to assume that this event was designed such that this is not possible and force the players to focus on a subset of items that are achievable. Personally I don't see anything wrong with a game mechanic where you can't get everything.
As for the mechanisms of getting pumpkins: I don't know if there is another weekly quest chain, but if not, it is indeed challenging to get many and you are forced to resort to a lot of adventures to do so. And looking at the list of adventures where one can get pumpkins, it is indeed an odd list:
Doing Co-Ops (I know BB want to encourage this, but the reality is that its rather cumbersome and not always worth the effort unless you are already part of a group that is geared to doing this regularly).
You are then left with adventures that may cost more than buying pumpkins straight off, or follow-up adventures which are difficult to get to begin with. Indeed leaving the list of do-able adventures very small (Black Knights / Stealing From The Rich for the most part unless you have a cache of others beforehand).
The downside of those of course is that you have to deal with loot-buddies (not always practical for casual players).
The overall experience for pumpkin acquisition could be made more positive if they added additional adventures or ability to acquire adventures, for example:
-- Add Outlaws or Bandit's Nest (not just Return to) to the list of pumpkin dropping adventures
-- Add Island of Pirates to the Shop
(This would add better variety and accommodate those who can not do enough/any multiplayer)
-- Ability to buy more pumpkin patches (at a much higher cost) -- or as a drop from adventures
-- More daily quests that give pumpkins
(This would give less a feeling of despair and allow one to dig deeper through their shopping list, but can still be throttled to prevent a full shopping spree).