cannot help you directly with any experience as I have never been to Alaska or used those lenses, but I would just take the Sony 70-400 lens and not bother with the others.
The Sigma 50 -500 apparently just falls short of a true 500 and the Tamron falls a bit short in image quality.
I still have my first DSLR, a Sony A100 it came with the 75-300 and its not exactly a lens to get excited about, why not leave it at home.
Not sure that an extender is really worth taking, it does affect quality and focus speed and the 70 - 400 gives you a pretty good working range.
Canon Powershot S95, Canon 7D, 15-85, Canon 85 1.8 USM, Sigma 30 1.4, 50mm 1.8, Canon 100 2.8L IS Macro, Canon 70-300L IS USM+Kenko 1.4 Pro 300DGX, Canon 430EX II and RS 4 Classic