In the original game, only one demon was allowed per hex. Moreover, a player may only use his Demon Dispell spell once per hex, so if he hits a stack, he may dispell the first demon, but after that, he's on his own. If a player is transported to a new space, he may use a demon dispell spell in the new space. Transporting also resets lair time. This can be quite important, as the Dragon's Lair sometimes fills up with demon stacks.
Fortnights are too long.
The first two fortnights, again, should be 14 days. Lair time should be calculated as if fortnights were 14 days. The number of tasks to gain a thwart should be a function of the number of players. Make it three times the number of players. As a result, a four-player game will usually see only two thwarts.