The fear wasn't extradition, it was rendition. People had been kidnapped from Sweden before. Sweden even had to run its own military ops to stop it happening. The problem is that parts of the government still seem to be cooperating with the US. The crackdown in TPB appears to have been at the request of the US.
It also seems highly suspicious that the case was dropped and he was told he was free to go, but then the prosecutor changed her mind and refused to interview him in the UK. If you were wanted by the US, if US senators were literally calling for your head on a platter, would you take the risk of going there? If you had seen leaked evidence that they do in fact render people to other countries, torture them for months, take them Guantanamo and torture them some more?
By all means lets have an investigation. The Swedish authorities want to question him. They can do that in the UK, or by video link. He offered, repeatedly, and it's been done before. Then they can decide what they want to do next, and we can hear some charges and legal arguments.