What are "salopettes"????????
You always have to pay for excursions up front. If you booked them with HAL and they are cancelled because of weather, etc., then your money will be returned as a credit to your shipboard account, I believe.
Can't help you with clothing for your Dog Sled excursion, as we have never done that.
Most people are out on deck, on their verandahs or in the Crow's Nest for the Glacier cruising. There are plenty of sunbeds so don't worry about that.
You won't need super warm clothing, just layer your garments. When we were in Alaska the end of August a few years ago, I wore cords, turtle neck t-shirt, heavy sweater and a winter jacket, without the lining - being Canadian I guess I don't feel the cold as much as other people. Oh, don't forget a hat (toque) and gloves or mitts. Make sure you have good walking shoes also.
The day we were in Skagway, in the morning I was dressed in warm clothes and by the afternoon it was so warm that I changed into summer pants and a light weight short sleeve pullover & light weight cardigan.