You don't really do anything in chapter five. You just go on to the next chapter.
Chapter 5-11 were released on the 18th September 2012.
Chapter 5 of Harry Potter and the Chamber of Secrets was released on the 18th September 2012.
No you can't. If it isn't an important chapter you can skip it, but if it is a compulsory chapter you have to do it before you move on.
Explore chapter one and then you should be able to move onto chapter two.
Chapter 5 was locked for a long time as it had not been released. It should be open now, if you still can't access it make sure you have completed chapter 4.
The Pottermore Wiki contains guides which goes though each chapter and also contains lists on every collectable item.
You have to finish all the stages in chapter 15 in order to move onto the next chapter.
Yes, you can find many things. For a guide per chapter see the Pottermore Wiki.
It is open now, you should be able to access it so long as you have finished the previous chapters.
You don't interact with the diary in this chapter.
Chapter 16 of Harry Potter and the Prisoner of Azkabanwas released on the 31st July 2013.
If you mean book two, apparently early 2012