This came up when we were working on this, the team at ReadMe has been very responsive about this and I don't feel like we'll be trapped on the platform. They use Markdown for almost everything except some widgets, and since I'm going to use mainly Markdown I don't feel locked in.

I also feel confident that if I kindly asked for the contents of my site, the team at ReadMe would send it on over in what ever way they could. That may change in a year, but I doubt it, if anything, I bet they'll have a more automated way to get content in and out.

The ToS sounds pretty standard for "content" sites... that is, they have to have the rights to display the content, since they are the host. In my experience you're much more likely to get a shady crawler copying your content and reposting it than the service you pay using it in ways you didn't expect.

edit: woops you deleted. I think your questions were very valid, so I'll leave my reply :)