Surrounded by BLM land. Have fun when they start drilling for oil! You'll wish then that you'd bought in the gated community where the only unsightly noises are leaf blowers and your neighbour sexually harassing his maid.
Find a drip... freezing level is high so there should be liquid water dripping, trickling, or running all night long.. look at rocks or the upper lip of moats and snow cracks to find liquid water and camp accordingly.
For all the BS about the Cascade Trifecta, the 'uncage the soul' guys seem to spend more of their time playing dodgeball. maybe dodgeball would help you mike?
There are limited sites on the spit. Most all of them will be taken by the time you get there. You will end up sleeping in a puddle and wake up with at least a dozen slugs crawling over you. Also bears swim across the river at night and attacked someone there last year. The Spit Sux.
If you want to dirtbag it, drive past the Hydro Project on the Mamquam road. At least 10 km.
There is some logging equipment in there but no active logging at the moment. Don't know if the road has been graded this year. Call Cattermole Timber in Chilliwack or May Trucking in Hope to arrange a key. You will have to try and pick it up by closing time on Friday which could be tricky if you are coming from Seattle, or even Vancouver.
25km sounds bunk. The first gate is at 13km and the second around 17km. The peaks are near 20km as I recall.
Anderson River Mountain itself is a lump of choss.
Borrow for peace of mind. The road should be in good shape as far as the left fork to the bridge and pumice mine. From there it's 2 hours of hiking with no 4wd if crossditched, or 10 minutes of driving with 4wd. So you save yourself 4 hours on the weekend with 4wd.