1) There is more than one stretch that can be impassable. Easy enough to plan around. Tide book -45 minutes from Shelter Cove will give you the approximate time for closer to Mattole
2) Yes. Fall in Humboldt is sublime(usually)
3) Yes. and your hiking permit is also your fire permit.
4) They are getting more brave as it is getting busier there. Bear canister per person is mandatory.
5) Oh well. It's about the bears, not you. But yeah, they suck.
6) They try to keep them from remaining. Some get built every year. Then volunteers get to take them all down again. Much better camping available up off the beach anyway. Sleeping on the beach is for the noobs.
7) Maybe none. Guaranteed to see one when you don't want to. Seen them hike people back to the trailhead. So they are there.
8) Surf fishing allowed with license
9) PM me
10) Yes, but they might never want to go again. It is not easy. The beach portions are on a slope, sandy, rocky slope. Up on the terrace life is easy. But not enough of that at times. But four days is way enough time. 3 days usually works well. 24 miles.
Can be done in a weekend if desired.