Good points econ21.
I actually also came across this but didn't have the time back then to write a lenghty argument about it. Wills really aren't covered in the Rules except for that half sentence. I believe the idea back then was that only land from an official Duchy is 'safe' from the King. Which was the basis for all of my argumentation. Although I didn't realize it fully at the time I'm glad that my support of c) is fully in line with that.

The question really is whether land can be given away in a Will. We had a stance on that in LotR, don't what our stance is here. I can see Bradens point where it's just sensible that anything you write in the Will is fulfilled. After all why should you loose a right (giving away provinces) you had while alive once you die?

On the other hand we wanted a strong King, which would be supported if land falls back to him from independent Nobles. So only land within a Duchy is hereditary, while outside a House/Duchy it will always fall back to the King. Which I think is more in line with history, where land was given with and without hereditary claim to it.

But that's really something for another discussion I think.