I've been on every road you've described, its going to be an awesome ride. Make sure you have the schedule sorted if you plan on going to skagway from haines, this ferry doesn't run often and its just to get to Juneau and back.

My other piece of advice is to not take the Stuart-Cassier highway up and down, at least take the alaska highway one way, in my opinion muncho lake park is one of the nicest places in canada and there is so much wildlife you will see, sheep, bears, cariboo, buffalo. Camping on the lake is amazing. The trade-off of course will be the boring drive from Fort Nelson to Fort St. John but well worth it in my opinion.

I used to live in Whitehorse and spent a lot of time in Dawson City if you have any questions. Its about a 5.5hr drive from Whitehorse to Dawson City and not a lot to see. If you have time, when you go down towards Haines, you can go .5hrs north of Haines Junction and there is a mountain called sheep mountain right by the side of the road, there is a parks office in front of it and you will see Dall Sheep all over the mountain in the summer. There is camping nearby 5 more minutes up the road (provincial campground), free firewood in the yukon! The dempster is a beautiful drive, hopefully the weather co-operates! Please post pictures if you get on the new section north of Inuvik, I don't know anybody that's been up there yet.

Cheers

Terry