Upload & Sell: Off
Well, although I live in Alaska, I've not flown my 500 on Alaska Airlines. But I wouldn't hesitate to do it since the lens would be in a strong case and carried onboard with me. All the flights you mentioned are conducted with full-sized jet aircraft and your 500 should fit in the overhead bin.
Now, the 100-400 is a very versatile lens, and I have one with me on the truck or Jeep seet most of the time, but if I were in Denali Park I couldn't imagine being without the 500! But then it depends upon what you require in terms of wildlife images, I guess?
I often get by with the 100-400 attached to a 7D, but I love my 500!
You are going to love Alaska! Be prepared for rain. It rains in 9 different directions in Ketchikan, and Sitka cab be rather rainy as well.