Sorry, I haven't read the whole thread but this is what I would take. Since it's only 20 days I would plan to do no firearm hunting, so my firearm would only be defensive, and it's a toss up between a rifled 12ga half sabot half buckshot and a .45-70. I like the rifle more but I think the shotgun is more useful.
As far as a buck blade, I would prefer a medium size axe. I don't know if they ever made one though. The 757 would work if I could re-profile it before I was dumped. Hopefully I have a sharpening stone of some sort. If I can't have an axe I'd take a woodsman or an filet.
I love the Rockies and it is a dream of mine to live near them in a way that allows me to enjoy them daily. I have spent four summers so far in BC, the most relevant (and best) part being a four week stay in Kwadacha park, near fort Ware just south of Haworth lake near Cloudmaker and Little Cloudmaker mt. (8,200ft elevation!). There are First Nation people there, the main ones being Sekani and Western Cree.
We weren't hunting, so the only firearms were defensive, and of course we brought in more than enough supplies, but I have no doubt survival could be done there, especially at this time of year, with the supplies suggested. I would set up lines set to various depths throughout the lakes as well as use wooden fish traps to supply fresh food. Many of the trees, such as maples birches and spruces offer sources of food, and there is a plethora of grasses. I would also not be surprise in the least to be able to find crabapple (boiled they become palatable), raspberries and I think we saw some huckleberries but I'm not positive. I don't think it would be necessary, but I would also consider setting up deadfalls on game trails if fishing and foraging didn't turn up much. My main meat sources would be fish and turtles. Shelter would be a wooden lean-to with leaf insulation and a sod/bark roof. Could be constructed in 2 days and will see you through significant weather. You can and almost definitely will have severe storms and snow if you are far enough north/high enough elevation so don't forget that! Shelter would be my first priority once I found a good site.
Anyway, I think I could go 20 days. I wouldn't venture far from camp (traveling through this terrain is rather dangerous... you can easily become seriously injured from one misstep up in the crags) if I didn't need to, and since I know I can sit on my butt for 20 days and get picked up I would do just that. As other have said I would love to take my dogs with me as I would feel far more safe and I think that would help me significantly.
If it were for more than a month I'd reconsider what I was taking and I would plan long term storage and game hunting. I'd also have to think it through again if I were to go up in a different season.