Loving these ideas! I think I've got the background set down now. I've decided to drop the undead component of the manor in favour of a full-out fey themed arc.
Our BBEG is a
Blackwood Satyr who has kidnapped several beautiful young women and is holding them prisoner within the labyrinth on the other side of a gateway to the First World. Our boss fight will end up being against him and three other regular
satyrs, for a total of a CR 9 encounter. He's been releasing the women into the labyrinth and then he and his companions chase them down for fun, with predictable results for what happens when they're caught.
The hedge maze was created by a
Blodeuwedd using their at-will plant growth SLA. This would mechanically allow a hedge maze to appear around the manor overnight. The background story was that our manor is owned by a nobleman that offended our fey antagonists somehow, and in retribution was taken prisoner to be punished. Thus the maze would function as per the
Plant Growth spell, meaning a PC's speed drops to 5 ft. within the hedge, and would likely block line of sight.
The manor itself is now ruled over by a pair of fey posing as the new lord and lady of the house. A
Stroke Lad and
Huldra have teamed up to take over the manor house, using their abilities to disfigure the servants into grotesque parodies of humans.
Obviously the manor house at the centre of the maze would be the first destination of the party, so assuming they get past the maze, they will discover it filled with horribly disfigured servants. Since Stroke Lads are supposed to be cowards, our fellow will reveal the location of the gateway to the First World where the women are being held prisoner if the party succeeds on an Intimidate check, by dropping him to half hit points in combat or by killing the Huldra.
I love some of your ideas, though as a DM I usually try to make sure any kind of effect has a basis in the core rules instead of hand-waving it away and saying that 'some strange magic stops you doing that' to prevent PCs using a trick to bypass the challenge.
I like your idea of the guard at the entrance to the maze being a test of sorts. I'm thinking a CR 5
Fey Touched Minotaur to introduce the party to the maze and fey theme of the adventure arc and also remind them of the need to have cold iron weapons to bypass the DR of these creatures. The big issue will be communication, since it would only speak Giant, but I figure if nobody has access to that language it would be simple enough to have an easy Sense Motive check to let them know it wants to test them before they can pass the entrance to the maze.
I also like your idea of seeding the maze with helpful Fey NPCs. In particular, I was thinking of having two
Baccae try to seduce the most likely PC and promise to give him a 'gift' if he stays to drink with them. In this case, 'winning' the drinking contest actually involves failing the Fortitude save listed for their special wine, which has no real harmful effect (though making the PCs roll Fortitude saves that increase in difficulty each round will probably have the players extremely paranoid).
As for the maze, I think I'll add four
Twigjacks that attack players who push through the hedge instead of following the path. A few Splintersprays should act as a decent deterrent to doing this, I think.
I'm not a huge fan of The Gambler encounter since, as mentioned before, I don't like creatures that have abilities that aren't part of their monster entry. Poltergeists don't have any ability that would let them take the soul of a creature, willingly or otherwise. I could probably do something like give them a Cacodaemon's Soul Lock ability, but overall that doesn't seem thematically appropriate.
On the other hand, the Blodeuwedd has a Lesser Geas SLA that could be quite fun. A Lesser Geas of "Leave this place and never return!" could inflict some discomfort on one member of the party, especially if they try to spend time resting after the fight with the Huldra and Stroke Lad to recover their ability damage.
Since the underground labyrinth is part of the First World, that would mean it would be unlikely to hold any undead or dragons (aside from Linnorm, who are all far too high a CR to place in this quest arc, even as an optional bonus boss). Still, I can think of a few good subterranean fey who might be good additions, as guardians or otherwise. A
Polevik or a pair of Tome of Horrors 4
Domovoi would be good ones, as well as a
Lampad to act as a red herring encounter to trick the party into thinking they've found one of the kidnapped girls.