1. Yes ..it does.
2. Will do you no harm if you have enough memory to spare. But the question you need to ask is - do you really need to ?
3. extended_statistics is used to justify whether an increase in the db buffers will be beneficial or not. I am doubtful if it increases the db buffers but it certainly does not increase the library cache. Remember the 3 main components of SGA :-
Shared pool (library/dictionary cache info)