Thats a a lot, it's possible, but you won't enjoy it, I would recommend less

For all of Alaska, Yukon, and most of BC you can 'stealth camp' for free. There is so much of nothing that belongs to noone. Granted, you will want to stay at campgrounds every few days since they have showers, laundry, electricity, etc. For a campsite you should budget roughly $10 to $15

That depends if you want to cook your own food or not, and if you do, you'll probably get sick of it soon, but I would say roughly $30 a day or so, which doesn't include bicycle parts or things like that

Not that I know of, there really isn't anything there. I would recommend getting very good maps and finding those guidebooks for RVs and things like that. There are towns on the map sometimes on which there isn't much. The Casiar highway is a good example, there was a stretch of about 400 km of 'nothingness', and where there was something it was closed for the season still

If you want to stay at a campground you can find a list of provincial/state campgrounds you can stay at, or if your on a tight budget just walk 100 ft off the side of the road
As for food, the only place your going to find anything is in the cities/towns, there isn't anything in between

The winds were very, very bad in some places, I specifically remember a place called 'Destruction Bay'. Your goal of 75 miles per day is doable, but your going to hate life. The Casiar highway is a little slower for biking since the road conditions are poor and the road is gravel in some parts. Additionally, roads in the Yukon are terrible, full of pot holes etc.

In my journal I usually detailed if I had a rough day or if I came across nothing, my best advice for you is to get good maps
Couchsurfing won't be much help to you apart from the larger cities