I hope nobody thought I was suggesting they actually do that. It was purely a matter of personal curiosity. Entropy is perhaps the most interesting thing in the universe.
I'm far too lazy to do anything other than slap bcrypt on it, unless there's a pressing need to do something else, which there never is.
(Not an expert) The entropy doesn't change. What you get by adding another hash function is a slightly larger die area (and thus the cost) required for brute force attacks. By using scrypt instead of PBKDF2, you can adjust the die area required for attacks in a more flexible and safe way by just giving it a different parameter.
Hash functions have different requirements than key stretching functions, but if you're interested in the security of combining cryptographic hashes, Google for "combining hash functions" and "chaining hash functions" -- there's a lot of interesting research.
Yeah, that now seems obvious. The input string is the same, so the information entropy is the same. I'm struggling to think of the concept that I need here, I want to say kolmogorov complexity but I know that's wrong too.
I'm far too lazy to do anything other than slap bcrypt on it, unless there's a pressing need to do something else, which there never is.