I spend a lot of time in the mountains, and a lot of time rescuing people in cold mountain environments, and I agree that it's all about layering, with a waterproof shell layer you can wear on the outside in bad weather. If your clothing gets wet from snow or rain, it's going to be hard to keep warm. I prefer merino wool base layers, with wool, fleece, and synthetic down mid layers (real down doesn't work well if it gets wet and there are some great synthetic blends available).

As for a bug out bag for the situation that you mentioned, you can fit a lot in a pack and still keep the weight down. Personally, I would include a small handheld backpacking stove with small size butane container in a lightweight cooking pot (or the full jetboil system if you have the weight). This allows you to heat/boil water, cook, start a fire easier if things are wet, and the weight is minimal. I also have some blackbeard fire starter in mine, and a small road flare can help starting a fire or signaling for help.

Something for shelter is important, so it's good the plane has a tarp, as you can do a lot with a tarp, mylar emergency blankets, and some paracord. A waterproof bivvy is nice if you have the weight, but not a necessity.

Outside of what you mentioned, i think a couple freeze dried meals and water can get you a long ways, and I always have a lightweight water filtration system like a Sawyer with me. I also keep a headlamp and spare batteries, a radio, a multi tool, and a little role of Gorilla tape that can be handy in a lot of situations.

The fact that you are already thinking about and preparing for a bad situation, will put you far ahead of the curve. We rescue a lot of people, and there is a big difference in outcomes between those who are prepared and those who aren't, and those who have the mindset to survive. Now that you are prepared, let's hope you never have to use it