Man..I am having a hard time figuring out your plans. For sure, go via 99 to Cache Creek. You can boondock in lots of places after Pemberton, once on the Duffey Lake Road.
If you want to go to Revelstoke, you can boondock in Chase at the Safer Foods. (free dump in Kamloops at the info centre) Grass lot right at the store. Shuswap 'docking is slim to none. However, in Salmon Arm you can at The Mall at Piccadilly..also has a free dump. Plus probably at CDN Tire. From there to Revelstoke is pretty slim boondocking. I'd say turn right at Highway 23..just before Revelstoke and head down to the free ferry at Shelter Bay. Cross on over and just toodle around the Kootenays till you want to head for Montana. Lots of boondocking to be found after you get off the ferry and if you like hot springs, you have two to choose from. The Nakusp ones have a campsite. At New Denver you make a choice...east or south...both great options. If you go left on 31a, Kaslo is wonderful tiny community. Then you just follow your noses on down to Nelson and carry on.
Gary Haupt