I'll leave some thoughts, with the caveat that I haven't bothered doing stuff like this in many years because the cost structure only makes sense if you're going to build 40 great sites (eg sites making more than a few hundred dollars per month), and I'd rather spend more time on fewer sites.
I think most of the people doing this effectively are probably using option number 1 or slight variations, and not wasting their time on building any sites that can't cover their individual hosting costs. In other words, if the new site can't cover an incremental $10-50/month hosting expense, it's not worth your time to build it in the first place. With this route, you can also centralize control of the decentralized sites with a tool like ManageWP assuming it doesn't leave public footprints (like /wp-content/plugins/ URLs), but it'd still be all Wordpress site footprints.
At the scale you're trying to do this at, it would be pretty easy to "blow your cover" with extensive interlinking of the sites, shared access to GA & GWT between any of the sites, shared registrar dates & contact info (or shared registrars at all), and like 20 other server & CMS-level footprints. I'll assume you've covered all of those bases if you're at the stage of testing ways to centralize your hosting, but it's worth bringing it up for anyone else who is trying to take this approach.
I would be skeptical that the CDN option would work. Unless I'm mistaken, the sites would still need to have the same nameservers before the CDN delivered any content, and any search engine could plainly see that.
Not sure about the Varnish method.
Most of the hosting providers you'll talk to don't want to give you class-C IP addresses, because they spend a lot of money to acquire more IP addresses over time, and the type of customers asking for more Class-C IP addresses are typically mediocre hosting customers.
Just my two cents. You probably won't get too many replies on here, because most of the people doing this effectively don't feel like sharing their methods. Blackhat World or Wicked Fire will probably get you more candid responses, but your question is pretty specific, so I don't anticipate too many people willing to describe their full stack or workflow in detail.