(1) Yeah, like them neo-Torquemadas will care if the images are in your computer's temporary or permanent memory (and depending on your Web browser's settings the 4chan images can stay in the latter for quite a while).
(2) In order to get legally-admissible evidence against you via your hard drive they need an arrest warrant, they need to find the hard drive, make sure it's not destroyed by the time they get to it, and then decrypt it. As storage devices increase in smallness, stealth, and self-destruct functionality, the latter three become ever-more problematic for the tyrants - it will soon be possible to store terabytes of data in a dental implant you can wipe clean with a few flicks of your tongue! This is why the fascists use centralization of the Internet for their benefit instead - if your ISP-assigned IP address is in the server / interceptor logs, then they have all the evidence they need to have their witch-burning.
(3) This is about maximizing proliferation of free speech, not making yourself immune to persecution.
Maximize the proliferation of free speech all you want, its still on your machine, and don't be so stupid as to think the source isn't a fed. Regardless of
your ability to out-fox the foxes, many people are less capable. If your download comes with a worm in it, you're fucked, and it takes a while for custom-made worms and trojans to be recognized. You think Norton is gonna spring into action if a worm starts showing up in CP files? Think again. Low priority. Meanwhile, it sends back to the mothership.
You are at the mercy of your software, whether you like it or not.
The solution is to avoid things the market will not make a fuss about. If some little haxxor worm was released and nine people got scooped up, nobody would even know. If the feds dropped a warrant on moot, a million people would know in five minutes.
The feds are perfectly happy to ruin one person at a time. They'll hand out medals all day long to their little nerds who run a sting on altnet, and result in four busts in six months. They have hundreds and hundreds of little geek cocksuckers doing all sorts of nasty, and you know it.