HTTP challenges do not offer meaningfully increased resistance to MitM over DNS challenges, and require measurements from multiple vantage points just as much as DNS challenges do.
No, but it is a good example that CAs don't require DNS changes to assign a cert. One just needs to MITM an HTTP connection for a single request. (Or, depending on the CA, control one of the following email addresses: "admin@example.com, administrator@example.com, postmaster@example.com, webmaster@example.com, hostmaster@example.com")
