I think if the Fraser Canyon Gold Rush and following gold rushes attracted a few more people then we could soon see a snowballing change in the history of [British] Columbia. Gov. Douglas very much feared the region would become a new Texas. Say he acts a bit rashly, a thing he tried to in OTL regarding the San Juan Islands, but the US doesn't take the bait (just replace Brig.-Gen. William Harney with some one like Meade or Thomas). This would make Britain the aggressor and a bit embarrassed. Once the ACW roles around the US agrees to renewed joint occupation of the San Juan Islands and gets a guarantee of fair treatment for American settlers in New Caledonia (the mainland of British Columbia).
Post war the issue is brought up again and the US quickly purchases Alaska. Seward presses the Alabama Claims but hints that perhaps a deal can be worked out. In return for the suppression of the Fenians, the US accepts ownership of New Caledonia and the Stickine Territories in lieu of payment regarding the Alabama Claims. Britain keeps Vancouver and Queen Charlotte Islands. So the US gets the entire mainland portion of British Columbia by no later then 1872.
Benjamin