Correct on both counts.. Pre-fetching DNS names for origins not on the page is, indeed, a nice use case - redirects are a good example. Similarly, embedding a <link rel=dns-prefetch> in the head will trigger an early DNS lookup, which can, in fact, deliver big latency savings -- you would be surprised how slow DNS lookups are in practice.

Last but not least, AFAIK, I'm not aware of any logic to group DNS-prefetch hints or similar strategies when running over TLS.