Icefields is a beautiful ride so hopefully it doesn't come to that. However, I'll note that you likely have some flexibility in sorting choices along the way:
1. At both Prince George and Tete Jaune Cache there is both internet and an alternate road heading southbound.
2. You might have flexibility if you needed to delay or shift things by a day.
3. There are multiple choices between campgrounds and hostels so if necessary can make a shorter day or longer day.

So that gives an ability at Prince George to do a second round assessment and look through combination of alternatives, perhaps shifting a day or doing a short hop if necessary. If it looks completely bleak then you can defer finalizing a choice until a few days before.

Alternately you could make some of those duplicate reservations earlier and again not months in advance but a week or two.

In my case in mid-August 2016 it was peak tourist season and I was fortunate in making reservations two days before entering. Things were mostly full and so not everything I tried was available but enough to put together a plan with reservations. I ended up spending a week in Smithers having a hub replaced (including replacement being mailed under warranty) so if I had made reservations much in advance of that, I would have had to change anyways or use the train to skip ahead.