I still sweat under the pack, even when stripped right down to just a tee shirt.
It is one of those places that just don't breath.

I would still carry long underpants as well as the outer pants.



True, true, but it beats getting chilled when you stop.
No sweat is a major rule in the northern bush.
If you sweat you freeze.

And if you are in a plane that goes down you are likely better to stay right there until they come find you.
If the pilot is competent at all he "should" have a better survival kit on the plane than you can pack, and the plane itself is a valuable source of material too.

edit.
I have never been there, but it should be flat coastal plain with few trees.
Possible extreme cold weather from the arctic coming through the Bering Straits, and occasionally temperatures might go as low as 50 below or colder, but normal winter temperatures should stay above 0 degrees F.

Later:
I was just looking at Bethel on Google Earth.
It looks like it is 55 miles up from the mouth of a meandering river. It also seems to be surrounded by at least 30 miles of muskeg in any direction and over 70 miles of muskeg in some directions.
One of the villages it serves has a total population of 6 people.

It sounds like a great trip, take a good camera!