This teams D is much improved and getting better. I can't see how there's much to complain about at the moment. They usually shut down the middle and just need to work on guarding the 3. Other than that this team is playing much better on D and using the same defensive system Mike has always used, but finally with some real defensive talent in it.
As for proving himself. There's only one thing left for Mike to do. He has to get to the finals and win. He's won COY, gotten to the WCF's. He just has to get to the finals and win, which is no different than any other coach. What had Doc proven until he won a title or Rick Carlisle? They were good NBA coaches and still are.