The failure of Canadian Confederation is...unlikely at best. The provincial elites (both economic and political) realized there was too much to be missed if they backed out of Confederation. The demographic and economic factors were pretty much all against it, and at the Charlottetown Conference delegates from every province were enthusiastic about the idea.
That being said there is a chance Canada could be split into three separate entities: a 'Canada' consisting of potentially Canada East and West and much of the prairies, a Maritime Dominion consisting of only the Maritime Provinces, and British Columbia which would probably find itself linked to Canada by the want of both parties to connect to the trade on either side of the country.
Remember that the original conference on Confederation was only the Maritime provinces. The Union of Canada (Upper and Lower = Ontario and Quebec) invited themselves to the meeting. If they hadn't, it's not terribly likely that the Maritime Union would have included Canada.
Note, too, that iOTL, PEI and Newfoundland both refused to join the new Union initially. PEI came around after a handful of years (AFTER BC!), and Newfoundland took until after WWII.
So, avoiding anything like OTL's Canada is pretty easy.
As for BC: It was accessed by sea mostly until the advent of the CPR. If OTL's Canada never happens, they could easily stay a British colony, ultimately a Dominion, just like Newfoundland did.
Prairies. There's a handful of ways to get to the Prairies.
1) You can do like the HBC did and come in through Hudson's Bay, which is only open a few months of the year, and then canoe upriver. No way are you are you going to get more than a handful of settlers in that way.
2) You can do like the NorthWest Company, and come in through Lake Superior, then paddle upriver, over portages, to Lake Winnipeg then paddle up river from there - following the same basic river routes as the HBC.
Neither of these methods lets you bring in more than a handful of settlers. And neither lets you export agricultural products. High value furs? Yes. Precious metals, if there were any, which there aren't? probably. Grain? No way on God's green earth.
3) up the Red River from Minnesota. Until the advent of the Railway, this is the ONLY way to get production out. However since this route goes through the US, the prairies would become first an economic colony, then a political one, of the US.
4) Building an (almost ASB level of) expensive railroad across the Canadian Shield. Something like a thousand miles of granite and basalt, interspersed with lakes and muskeg.
If that railway doesn't get built, as it very nearly didn't even in OTL, then the Prairies WILL be settled by Americans. Once the population is majority US citizens, well, Britain's not going to go to war over an area that a) doesn't want them, and b) they can't get an army to.