1. After mentioning the constant surveillance, Winston says that there is nothing of your own except "the few cubic centimeters inside your skull". He refers to the fact that Big Brother still hasn't found a way to sneak inside people's brains, in spite of all the brainwashing techniques. It also proves that Winston still isn't brainwashed - that piece inside his skull is still his own.
2. The phrase "mutability of the past" is ironic because our common human experience teaches us that past cannot be altered; once something is over, it can't be changed retroactively. But the totalitarian regime strives to do just that - to change people's past experience, and not just their present and future.
