I think the route you are talking about is the Old Island Highway. This is a very scenic route that has much less traffic than the new Island Highway. You can use bike paths and/or back roads to get from Nanaimo to Lantzville. Then for a short section to just south of Parksville you would have to use the new highway. It is not a difficult or particularly dangerous section to ride. The rest of the ride to Campbell River is on the old highway and much of it follows the coast.
The following link shows you the bike paths in the Nanaimo area.
http://www.biketowork.ca/files/Broch..._map2009_1.pdf The ferry to Gabriola Island leaves from the harbour. It is listed on the map. I have not cycled on Gabriola but i remember that there is a rather steep hill right up from the ferry terminal. It is not likely to be dangerous to cycle on the island. There is very little traffic. I haven't been on it for a over twenty years and at that time I wasn't thinking about cycling it so I can't tell you if it is easy or difficult to ride.
Between Nanaimo and Salt Spring you can also use alternate routes for much of the distance. However I have ridden on the main highway between Victoria and Nanaimo and found it relatively easy and certainly safe. I have never ridden on Salt Spring but again remember it as having some decent hills like almost all of the Gulf Islands.
When you are talking about the Fraser Canyon are you talking about the section between Hope and Lytton, which is the real canyon, or are you also including the Fraser Valley between Vancouver and Hope? If the former, it is a beautiful route that I have ridden safely several times. The main challenge is a series of tunnels between Yale and Boston Bar. Only two of them are long enough to cause real issues and for both there is a warning light you can push before you enter. There is also a narrow sidewalk you can use to push your bike though the tunnel if you are worried.
If you are talking about the Fraser Valley there are routes out of Vancouver on both side of the Fraser. You cannot use the Trans Canada Highway until just before Chilliwack. Near Vancouver the routes are mostly on roads that can be fairly busy. I would not be riding on some of these roads during the afternoon rush hour which lasts from about 3:30 to 7:00. Once you get past Maple Ridge on the north side things settle down a bit and you can use the Lougheed Highway to Hope. I find the south side of the river actually easier. If you are coming from Tsawwassen ferry terminal you can use Highways 17a, Ladner Trunk Road, Highway 10 and the Fraser Highway to get you to Abbotsford. Then you would use back roads to past Chilliwack and then on the Trans Canada to Hope. With a little map work you can find parallel roads to these highways that work even better. I have a more complicated route that works well for me. If you are interested let me know and I will try to send the info.