And those criminals went to jail right? </s>
That such people go on trial needs to go hand and hand with any pardon. And this works both ways. That is, either Assange testifies against them at length, or they likely cannot get convicted. But until he testifies against them, he is a dead man walking for fear that he testifies.
The best bet for him is to give pre-testimony to as many people as will listen, assuming that what he says gets published. That is, he cannot just go in cold. He must do as much as he can to name names and weaken the powerful before he dares to leave that embassy.
Oddly enough, his biggest allies in this might be Edward Snowden and the Russians, all ‘fessing up at once, a huge airing of dirty laundry.
Assange and Snowden can identify the crimes, and the Russians can identify the people behind the crimes, if not showing evidence themselves.
Once they are identified as individuals, they will be much easier to take down, as these agencies have to sanitize themselves from the smell.