@abh12345 published recently statistics about that : https://steemit.com/utopian-io/@abh12345/who-holds-the-vests-that-are-not-voting-for-witnesses. This doesn't cover all your questions, but it covers a lot, and some more.

Problem is we don't have a SteemSQL subscription for now, so it's hard to make historical statistics with only the API.
But we will make statistics about the estimated impact of our initiative (first one will be tomorrow).