Tasks and missions are considered "items" as per the Dunwich rulebook (Page 5):
Tasks are a type of item found in the Common Item deck, while Missions are items found in the Unique Item deck.
If you receive an effect that requires you to discard items you can discard a task or mission as part of that, noting in the base rules (page 16):
The basic monster movement rules state that monsters don't move when they are in the same space as an investigator.
There isn't any reason to think that the stalker addition in Dunwich changes this in any way.
There is no maximum number of dice you can roll - it is infinite as the rules never explicitly call out a maximum dice total.
Considering the mechanics of the game though the extreme theoretical limit would be somewhere over 70 (but likely less than 80).
I don't have the time to go through every card to work it out, but given the example ...
It's possible to houserule anything, and these cards aren't an exception.
Just have in mind that if you want to add these in using just the Dunwich Horror rules, it will decrease the game's difficulty considerably.
In the core game when your Sanity or Stamina drops to 0 you:
Discard half of your items
Discard half of your Clues
Recover 1 Sanity or Stamina
The wording specifies that this action is in addition to raising the terror level.
The rules does not state that you add one Dunwich Horror token for each monster added. After increasing the terror level, if there a fewer than 3 tokens, add one token only