I always thought that Beren was working inside the oath of Feanor by claiming a Silmaril even if for Thingol, because what motivated Thingol to ask for the Silmarill was Beren. But that is a circular arguement I suppose...
Yes, Beren would have been subject to the Oath of Feanor, Samwise, because that involved anyone who kept a Silmaril from the Feanor or his sons. Hence the trouble he had with Celegorm and Curufin.
Do not confuse the Oath with the Curse of the Noldor though. Although the Curse of the Noldor came about because of the Kin-Slaying and the defiance shown to the Valar, it was all tied up with the fate of the Silmarils. I think in a way Mandos's Doom seemed to spread to anyone who willingly went out of their way to covert a Silmaril. In this Thingol was found guilty, and thus d@mned himself. Beren, however, was after a Silmaril as a means to an end, and therefore innocent of this Doom until he tried to take a second.
2) What did Melian mean when she said to Thingol, "...it is ill for you, whether Beren fail in his errand, or achieve it. For you have doomed either your daughter, or yourself. And now is Doriath drawn within the fate of a mightier realm."
All of your answers to this are as I see them. Either Luthien will die of grief if Beren fails, or Thingol will invoke the wrath of the sons of Feanor if Beren succeeds. Who do you suppose the Mightier realm is that Melian refers to? It seems strange she would call the Noldor this unless she is calling them it to emphasise to Thingol just how great a problem he has involved himself in.
1) Why was Beren able to pass through the Girdle of Melian, and why did both he and Luthien fall in love with each other at first sight?
Yes, it is fate. I believe Beren is one of the themes Eru wove into the Great Music to thwart Melkor. Melian had even seen this when she first wove the Girdle. I like your idea of Beren lost in his wonderings though, Arwen. If Eru is controlling Beren through a theme of music, he perhaps cannot just pick him up and place him where he wants him etc, he maybe has to do things subtly (even though he is God). Maybe the terrible misfotunes Beren had suffered were instigated by Eru just as a means to an end, the task being to get Beren through the Girdle.
6) Why were Beren and Luthien allowed to leave the Halls of Mandos after their deaths?
If Beren was a theme woven by Eru, we must ask to what purpose? It wasn't just to steal a Silmaril from Morgoth's crown, even though it did give the Free People of Beleriand hope when they really needed it.
A look at the family trees in the back of the book will show you just how important this union was to the future of Middle Earth, even though it was not mentioned in this chapter. When Luthien chose the life of a Mortal, it was said to her that her line would never die. A look at the family tree shows the descendants of this union to be Elrond as well as all of the Kings of Numenor, Arnor and the Chieftains of the Rangers of the North. A very important line, instrumental in Middle Earths future struggles against Sauron.
How does the choice Luthien was given relate to the choice of Elrond, Elros, and Arwen in LotR?
Luthien, through love for Beren, sacrificed her immortality. Had she not done this, and had married an Elf and had children, those children would have been immortal. I think this is why Elrond, Elros and Arwen were all given the choice themselves. Taking away the Gift of Man was not in Mandos's power so the choice fell on the individual. Even though Elrond chose the Elven path, his choice could not deny the Gift of Man to Arwen either, because if Elrond had chosen the Path of Men, the Gift would have been hers.
The choice does not seem to have been given to Elros's line. Once he chose to accept the Gift of Men, that seems have been it for his descendants too, even though many of them yearned for immortality. To me, this indicates that the Gift of Man superceeds Elven immortality on earth, because once accepted it seems to be the fate of your children too.
4) What is the nature of the creatures,
a) Huan the Wolfhound
What I was trying to highlight here, Thingol, is what are the true natures of these creatures. There is no way for example that Huan is just a hound. Anyone have any ideas? Is everyone happy with other aspects of this chapter, or have any other points they wish to share about it?