Very cool design. I like the "hidden" house a lot.
Some specific feedback:
The green roof as designed is cool, but will probably need a full railing on the edge. Might affect the "hidden" effect. Option might be to elevate the roof above grade 4 feet on north side so no railing is needed on a roof, and less excavation overall. But that would subtract from hidden effect...hmm.
Consider eliminating lower deck to allow more natural land cover (the upper is probably plenty, and nice shape!).
It'd be nice to have a view of ocean from some "public" space, be it dining, kitchen or living (as set up now looks like the den-bedroom 1 wing obscures it).
The entry stairs are cool, but worry a bit about claustrophobic feel, since at entry to house, you are in a 3-sided "box". When standing at the door, I'd like it to feel a bit grand. I also like some protection overhead on any main entry door to shield guests from rain.
Consider an Arctic entry.
Think about wall space in the den for a desk.
Don't need 6 double doors onto deck. I'd eliminate the one in den at least, and probably bedrooms to reduce heat loss. I see no need to exit bedrooms to deck given the multiple other options. I also like cool/dark bedrooms for sleeping, so sun exposure less important there.
Kitchen-living-dining arrangement looks very nice!
The space between kitchen counter and laundry seems a bit odd. Maybe move laundry room to this space (in cabinets) and shorten plan by the current hall-laundry area (and small enclosed deck space with E and W windows facing each other). Simpler construction, less perimeter for heat loss.
Or step master bedroom wing back 4 feet further into hill so dining room can have east facing window that looks at mountains (and gets morning sun)
I'd like person working at sink to be facing a window, or facing the center of the room. Maybe switch sink to the Island?
Reading room and den might be combined somehow to cut costs (multipurpose rooms).
As laid out, there is a hallway between reading room and den that goes under the stairs. It appears to get little natural light, which may be uninviting.
All guests in main house will have to head for the bathroom at dark corner of this hall. Can it be moved more central (maybe swap with reading room?).
Hope this feedback is helpful. Very cool design, and much along what I was seeking. Thanks!