Congrats on the new cabin - never seen Fishhawk in person, but pictures and reports of it make it sound like a pretty spiffy private fishery. I am a little surprised it opens during the late stream trout opener, since it's a reservoir. Do salmon and steelhead make it through a fish passage into the lake? Only reason to open so late is for out migration of smolt.
The Nehalem itself has a very good trout fishery for resident rainbows (or if you will, steelhead that say "screw dat" to migrating to the sea and decide to stay fresh with their resident cutthroat cousins) plus the coastal cutties. Has a good run of sea run cutts too. If you're fly fishing - try swinging soft hackle wet flies, it can be a kick for those cutties. Nymphing works well too.
Would love to see some pictures of the area that come from something other than the HOA website. The thing that surprises me - they say there's no gas motors on the lake - man powered or electric only, but they have a photo of a family enjoying time on the lake in a party barge (big pontoon boat) - never seen one that can be pushed along by an electric at anything other than snail speed. It looks like a GREAT lake to fish from a belly boat, kayak, or personal pontoon (cataraft type, not party barge type).
I would try my hand with an intermediate sinking line, and some woolly buggers in there. Black, olive, crimson, and brown would be my first choices. Same setup will work in the Nehalem too - them fishies eat buggers pretty well.